It depends on where you are in your running and where you are in your lifting and what your goals are for each. When I was still new at both I was making fast progress running three days a week and doing stronglifts which incorporates squats three times a week. When my running miles started increasing I had to drop back to doing squats once a week but was still making progress in both. Right now I'm focused on getting faster before a half marathon in 11 weeks so I've cut my lifting back to twice a week - one session with squats.

Lifting in the lower rep range will increase strength without too much increase in actual muscle mass. If your goals include being faster then being stronger is more important than being bigger. Compound movements with heavier weights will help you with this. A strong core is also important for running and will help with your posture as the miles get long.

It sounds like you're not doing a lot of lifting right now so you would likely make some pretty fast gains. Depending on what you're currently planning in your running I'd maybe take a look at Starting Strength. If the squatting gets to be too much and you can't recover well then you could reduce to squats only once a week instead of every workout.

It will take some trial and error to figure out what will work for you and tweaks will need to be made as you progress in both lifting and running.

Lifting in the 1 to 6ish rep range will focus on strength
6is to 15ish will focus on hypertrophy
Over 15ish is for muscular endurance.

There is overlap in these ranges.

You're already building endurance with your actual running. Starting Strength has you lifting 3 sets of 5 which is a great place to start for building strength. And yeah, squats the day before a long run is probably not a great idea

I did 9 sets of clean & jerks Friday and ran 14 miles Saturday - that was a bad idea and I won't be repeating that. And it's one of those things I should have know but.....

...and Tufel is a great resource and lucky to play tennis on most days!

I am currently following the New Rules of Lifting for Women program and I absolutely love it. Currently, I only lift twice a week, but was doing it 3 times a week when I was in the early stages of the program. I had to cut down to 2 times a week because the weight workouts increase in duration as you move through the program and it was too hard to work in a 3rd session with my runs. This program focuses on compound lifts and using all muscle groups in the same workout.

My schedule looks like:
Sat - Long Run
Sun - Lift (sometimes I will also run 1 mile to the gym and then back, but the mile home is pretty slow)
Mon - 3 mile easy run
Tues - 4 to 6 miles intervals
Wed - 3 mile easy run at lunch, then lift after work
Thurs - 3-6 mile run with strides or a tempo run
Fri - rest day

Here's a slide presentation from the Furman Institute of Running and Scientific Training (FIRST). Good bullet points, IMO.

www2.furman.edu/sites/first/Documents/16_oct2324.pdf‎

I just have to reiterate what sjohnny has just said.
Many running coaches now recommend some cross-training, particularly weight lifting. Strength helps. Gym time is training without the same, repetitive stresses of running.
The simple lifting routines that use mostly compound lifts are pretty fun and effective. You'll build strength, and, supposedly, you can build strength with this kind of lifting without packing on huge amounts of muscle weight.
The only thing is: there is a catch, or a trade-off, really.
On the days you lift heavy like that, you probably are not running. You're not running well, anyway. The programs like starting strength call for lifting three days a week. That means, if you follow that prescription, and take two days of rest per week, you only have two days a week for running. That is probably not enough to be good at running; it is certainly not enough to get better at running.
It is even worse if you take into account the fact that you are beat the day after heavy lifting so you run even less well.
I now lift -- compound lifts: squats, deads, power cleans, bench, overhead press, rows, pull-ups -- once a week, maybe twice. I figure that is about all I can do without impacting my running. But, I also did spent a fair amount of time lifting more often than that -- so I learned the lifts and my strength is already okay.
Good luck!

I always run before I do any leg work. I find that my legs are nice and warm, and I don't seem to have any trouble getting through the strength rutine. I don't do anything too radical though - it's a kettlebell leg workout using 12 lbs. Lots of single leg lunges, etc.

Yes, Friday is my rest day. I always try to do at least a little light yoga, but sometimes it is nothing at all. This is the schedule I shoot for. It doesn't always work out as planned either.

It depends on where you are in your running and where you are in your lifting and what your goals are for each. When I was still new at both I was making fast progress running three days a week and doing stronglifts which incorporates squats three times a week. When my running miles started increasing I had to drop back to doing squats once a week but was still making progress in both. Right now I'm focused on getting faster before a half marathon in 11 weeks so I've cut my lifting back to twice a week - one session with squats.

Lifting in the lower rep range will increase strength without too much increase in actual muscle mass. If your goals include being faster then being stronger is more important than being bigger. Compound movements with heavier weights will help you with this. A strong core is also important for running and will help with your posture as the miles get long.

It sounds like you're not doing a lot of lifting right now so you would likely make some pretty fast gains. Depending on what you're currently planning in your running I'd maybe take a look at Starting Strength. If the squatting gets to be too much and you can't recover well then you could reduce to squats only once a week instead of every workout.

It will take some trial and error to figure out what will work for you and tweaks will need to be made as you progress in both lifting and running.

Great advice and about all I can add from my own personal experience is that timing & scheduling can make a huge difference. It's a balancing act for sure. You dont want a hard run or a hard leg day close to each other. I also think a lot comes down to you running training philosphy. If you are marathon training where many of your runs are easy and not much speed work, it's going to be easier to fit in your lifting. one thing that has helped me is being able to lift before work and run after. That way even if I do some leg work, I'm able to run, unless its a hard run day. Which it usually isn't.

I like incorporating the strength training DVDs from P90x into my running program. They are mostly body weight exercises, like pull-ups, push-ups, lunges, body weight squats, etc., so I've found them to work well. I always do them after my run for the day and will do 1/2 of the DVD if I'm feeling too burnt out. I typical week with these looks like:

Mon - chest/back dvd
Tues - 5 miles
Wednesdays - medium long run and then shoulders and arms dvd
Thursdays - 5 miles and then legs and back dvd
Friday - Rest
Saturday - Long Run
Sunday - Recovery Run

I've found the key for me is to do leg lifting right before a rest day so that it doesn't affect my running.

Great, thanks for the feedback. Doing one of the workouts before and the other after work (or classes in my case) is a good idea. I'd be more inclined to do a morning run and lifting after class though. Is there a reason you do lifts before?

Its just more convenient to workout at home before work, and then stop at the park on my way home to run.

And for me it's the opposite. It's usually better for me to run early in the morning and lift in the afternoon when I get home if I'm going to do both on the same day. Totally a scheduling thing. As long as there is some time between workouts it shouldn't make a huge performance difference. If you're going to do them back to back you'll have to decide which you want to do first based on your goals and what effects doing one first has on the one you do after.

I'm hoping to incorporate long distance running with strength training as well and my schedule is hopefully going to look like this:

Monday - 6 miles and lift - upper body
Tuesday - 20-60 cross train and lift - lower body
Wednesday - 8 miles
Thursday - 20-60 cross train and lift - upper body
Friday - Long run (8-10-10-12 Miles)
Saturday - Rest/walk the dog
Sunday - 20-60 cross train and lift - lower body

I'll adjust if needed but hopefully the cross training will be enough to keep me maintaining my mileage with out always having to be running.

I really really prefer to run:) But, I think for me, strength training/cross training is important. 3 years ago, all I did was run and my knees began to bother me. First it was one knee(burning constantly) and then the other one because I was trying to compensate for the other. They bothered me even while walking. I remember going to the dentist and him telling me to keep my legs still(they were hurting). I never went to the dr. I just assumed my knees were saying no more running.

Fast forward to last January. I started exercising again(and relosing 30 lbs) and did Zumba, Kickboxing, Strength Training, and Spinning. In June, I cautiously started running loosely following the C25k program. I found my knees could handle it!

Now, I do 1 or 2 Kettle bell classes a week. This includes a lot of squats, lunges, core work, etc... I think its helping me run pain free so I plan to keep it up despite how much I'd rather just run.

Mon-Kettle Bells
Tues- 5.4 mile run
Wed-Am 3-4 mile run, Kettle Bells
Thur-Sometimes a 5 mile run(depends on where I am in my training)
Fri-Rest
Sat and Sun-A long run on one day(8-11 miles) and 5.4 mile on the other.

Way late, but bumping this as a reference for when I redo my strength/running schedule!

Thanks for the great answers!