Where is the line between hurting yourself and getting stronger?

Didn't make it to run club last Sunday because our car was stuck in the snow.
Put in 2 hours on the stationary bike instead.

Thursday: I did the ultra-conditioning dvd and felt pretty good. Definitely improving from the first time I did it. I'd like to try and do this once a week. I love the benefit of spin with strength training.

Friday: Ran about 5km at lunch. My legs felt a bit tired but not a bad run. To get back to work I walked the 200 stairs at the Hotel Mac. At the time I was tired but still ran back to work. That afternoon my legs just ached. I think the stairs were a little much. Had to skip swimming on Saturday because I needed to recover.

Sunday: Couldn't go to run club because we didn't have a babysitter. I did 90km on the stationary bike. Probably a little much this early in the training program, but I needed to prove to myself that I can bike 90km.

Today for the first time I actually think I can finish this half ironman. I'm going to be walking a lot of the run portion, but I do think I can do it. Hopefully I have a good race at Coronation.