I ran a 10 miler in Aug but fell off the wagon and have gained back a fair amount of weight. I'm trying to get it back on track but recurring back issues (sciatic nerve) and new shoulder issues are limiting what I can do.
If you plan on doing one of these, train outside, no amount of elliptical, treadmill, exercise bike, etc can prepare you for walking, jogging or running outside. The varying terrain, inclines/declines, wind in your face etc all affect you and will kill you if you are not prepared for it.
My girlfriends sister kept bragging about how many hours she spends on the treadmill every day.....She didn't make it up the first hill of a local 5K we did. Big difference pacing the moving track vs. propelling yourself over the ground with inclines and headwind.
I started just walking around the parking lot at work, increasing my speed as I felt able to til It got to easy, then I switched up and started finding loops around my office. Lot's of subdivisions/condo's around so lots of possible routes off of the main drag. By the end of last summer my daily lunch time walk was 4-5+ miles @ around 14min/mile.