I am totally new to running. Like... the "couch" in C25K new.
Time & environmental considerations make the treadmill a necessity (no safe outdoor running spot that doesn't require a lengthy drive).
Is treadmill running better than nothing?
And I know marathon training on a treadmill is difficult if not impossible, but what about a 5k? Can I reasonably train for a 5k (not planning or wanting to do anything other than not finish last lol) on a treadmill exclusively?
Yes, treadmill running is better than nothing. I would think you could do a 5k while only training on a treadmill. I have done it before last year. Your going to find it harder typically to run outside due to weather and other conditions such as hills. Also, on a treadmill your not generally working as hard since you don't have to generate your own pace. But, it can be done. I wouldn't try to do anything longer than a 5k when only using the treadmill though.
While I agree with pretty much everything jsimms435 wrote, I have to say this; for every rule there's an exception.
Personally I avoid dreadmills like the plague, however, I know two folks who trained exclusively on treadmills over this last winter (it was pretty rough up here in New England); both individuals logged between 70 and 90 miles per week on the mill, and both competed in the Boston Marathon and logged times in the low 3-hour range in spite of the fact they are both within a few years of being 60.
Fat old man PRs:
Treadmill running is certainly better than not running at all. I assume you have a reasonable quality treadmill so you can approximate a normal running stride. That said, a treadmill is a much more controlled environment than the roads. So there are some disadvantages to treadmill running:
@ 5K: Ontario Mills 5K, Ontario, CA, 25:17
New Balance Palm Springs 5K, Palm Springs, CA, 24:32
Angels Baseball Foundation 5K, Anaheim, CA, 24:24
@ 10K: LA Chinatown Firecracker 10K, Los Angeles, CA, 52:15
Great Race of Agoura - Old Agoura 10K, Agoura Hills, CA, 51:40
Fiesta Days Run, La Canada, CA, 49:57
I did my first 5K last April and my 2nd in May almost exclusively training on the treadmill. I think as long as you are reasonable with your expectations and realize you need to be slower in the real world you'll be fine. During my 2nd 5K I strained my calf and it took about a week for it to recover.
I was forced to the treadmill for all my training for my most recent 10K and it totally sucked. I think once you progress past just starting out that you really need to run outdoors or you run the risk of not properly strengthening your legs and feet. It has been a bit of a mental challenge because I live in an extremely hilly area but once you get past that hills really are your friend when training.