From what I've read, yes, for two reasons. 1) The treadmill cushions your landing more making it easier on your feet/legs/joints. 2) Something to do with it pulling you means you aren't doing as much work to propel yourself forward. I've also seen talk about no wind resistance on a treadmill but don't pay attention to that because I jog so slow I can't imagine there's that much resistance lol.

I'm doing an 8 person marathon relay in 2 weeks so I've been doing all my training outside so I don't go into shock the day of the relay by finding it much harder outside. Except yesterday morning when it was raining and I ran at the gym...boy did I have a hard time with my "mental toughness" since nothing was distracting me from how bored I was, and how I was sweating a lot, and oh god its only been 10 min I have a 20 to go, and am I running wrong and look ridiculous, and the kazillion other thoughts that try to sabotage my runs. Being outside helps me with that for sure because I'm distracted by the scenery.

I'd say do what ever works for you and your goals. If I didn't have the goal of running my 3.25 mile leg of the relay I'd probably have been doing a mix of indoor and outdoor (my dog likes the runs). And I'm sure if I was doing more indoor, I'd find ways to distract myself. I also can't imagine what it is going to be like to run outside in the summer...so I may be singing the praises of treadmills in another month or so!

In any case, enjoy C25K. It definitely gets easier to do the longer runs. In the early weeks, it seems like weeks 5 and on will be impossible. I'm on W8D2 tomorrow using the distance goals (2.75 miles). I've deviated from the plan a bit because I'm still walking occasionally, but I try to keep that to under a minute. The marathon relay is on the 13th. I'm pretty sure I'll be 90% running, 10% walking - which is still pretty damn great considering I wasn't running at all at in February. My goal is to finish in less than 45 min.