I've been taking a really conservative approach to getting back into the fitness thing. It's really helping me figure out my limits, and build back up in a properly gradual fashion.
I'm up to about 5 miles on my bike at a go, and today will be trying a 700 meter swim, and am using the duo of swimming and biking to ensure I have a balance of strength and endurance in both halves of my body. My new plan is to attempt to increase my rides by 1 mile each ride until I hit 8 miles, then 2 miles per ride until I hit 20 miles, at which point I will resume normal bike usage. For the swims, I'm increasing by 100 meters each swim until I hit 2000 meters, at which point I'll begin re-integrating lifting into my fitness routine and take swimming down to 1-2 days a week.
Failure, in any fitness routine, is something you have to accomodate for. Personally, I usually am pretty into weight lifting and failure is built into every single powerlifting strategy. When I fail on a lift, I then de-load (reduce the weight) by 10%, and work at that level with higher repetitions for a couple workouts. Then come back and re-try the goal weight at the regular number of reps and you SHOULD get it.
I've never really applied this kind of thing to my cycling but now I'm sort of forced to. Cycling for the last few years has been all about transportation with fitness very much as a side benefit. But now that my cardiovascular capacity requires strengthening from the ground up I am taking the strategies I've learned from lifting and applying it in new ways until I reach my goal of being able to comfortably cycle 20-25 miles (and I might take it further to enable myself to complete longer rides). I'm offering this on this blog not only because it helps me clarify my own thoughts, but for many new riders the idea of mastering the distances necessary to competently get around is a scary part of getting started. Having a goal for each ride, but a plan for what to do next if you fail allows you some breathing room. Push yourself forward, but allow for the fact you're human.