When I'm out of the bike and want to start riding back I found out some lessons: Even after a 6-month hiatus, you need to check your bike fit (saddle height at least), there's plenty of online info on that topic. I found the rubber brooks cambium saddle the most comfortable saddle for long rides (even with an untrained ass). Riding just 10 to 15 minutes daily for a week (and nothing more) works miracles on your rear resilience with your saddle. My endurance up on the bike wasn't determined by my cardio (if you start to feel tired, you can just ride slower) but for my back and arms (postural pain)... So... some pushups back exercises and ab exercises during the "adjustment" time improved my endurance a lot. A friend of mine who is a pro, told me once: "you don't ride a bike to get fit, you get fit to enjoy riding a bike". Totally true.