The thing that kept screwing up my exercise regimen was external factors like weather, overcrowding, etc. The exercise itself is already hard enough. Fighting a crowd or a blizzard makes it even more difficult.

I found that an indoor erg (rowing machine) to be the best way to solve the problem. I have total control over my environment. I know exactly how much it will suck and there is zero anxiety about getting to the gym before a certain time, checking the weather, etc.

Effectively, I have eliminated all potential excuses for not doing the exercise. The only remaining one is tired/sick/etc., but as long as I draw breath I can move a make believe rowboat at a non-zero speed.