Running injury?

Awesome running!

Rest, stretch, swim, and if you don't already, consider strengthening your quads and hamstrings. If it doesn't feel better see a professional.

If walking feels ok I would suggest this too.

Well first congrats on deciding to run outdoors. It really is a different experience. My make sure you are stretching really well. The impact on your joints outdoors is very different from a treadmill. If you start to feel pain, ice it. If you feel pain for more than 4 days you need to see a DR. I've learned that from experience and make sure you have the right shoes.

Google "Runner's World - Build a Better Knee" and do those strengthening exercises religiously. Also, there's a really good stretch workout by user "FitnessBlender" on YouTube called "Yoga inspired stretch and cooldown." Those two things minimize running injuries for me.

I would guess that you have a treadmill stride that you are naturally applying to road running. Try picking up your feet a tiny bit more to get a full range of motion with you knee. Also, the treadmill has a softer impact than the road and will be more forgiving on shoes that aren't best for your gait/strike/pronation. Have you been fitted for shoes?

Roy, the "experts" differ, but many believe that road running is more strenuous than treadmill running because you need to actually propel yourself down the road by pushing off more with your feet. There's also the headwinds issue that is often raised. That's why you'll see recommendations where they'll tell you to set the treadmill incline at 1% or thereabouts to more accurately simulate road running. I've done both, and have always felt that treadmill running was easier for me. As others here have suggested, i would concur that you should just take it easier - slower pace, reduced distance - as you transition. Happy running!

I've been running 10 miles per week on a treadmill for a month now so I've racked up some miles. On Saturday night at around 11pm I was bored indoors so decided to run for the first time outside. I ran for 25 minutes (No idea on mileage) and felt GREAT.

I mean really good. I wanted to carry on but thought ill stop now and do this 3/4 nights per week. I thought of a spot where I'd stop just outside my flats which I did. The moment I stopped my left knee felt 'different'. I really don't know how to explain it but it was the SECOND I stopped running. Yesterday and today (So 24 and 36 hours after the run) my left knee 'clicks' with every step I take. When I bend it and then straighten it again it clicks. There's no pain as such just a lot of clicking.

What do I do? Just rest?

there are a few things that could be happening, One you could have a weakened IT band if you haven't properly stretched or cross trained/strengthened this part of your leg. The IT band is what keeps your knee in place. This could have started on the treadmill but since the treadmill does part of the work for you, it may not have been an issue at first. But since you went outside you were forced to do all the work and the IT band became even more aggravated. You can google for appropriate stretches. Also make sure you are cross training and alternating your distances/paces/routines. Do short runs during the week with 1 long run on the weekend. Also make sure to properly cross-train and rest. Some of the biggest new runner mistakes is maintaining the same distance at the same pace for all time. You make no improvements by doing this. Also as everyone said before, get fitted for a proper pair of running shoes. All running stores do it for free. Give up the treadmill too - if you are planning on running a race at any point, you can't control what race day looks like and need to be prepared for everything.