I started riding last year and made some great progress just riding. This year I'm trying a more structured training plan in preparation for a big ride in June. I suck at hills (weighing in at 335 lbs. doesn't help), but this past Sunday I went and did some hill intervals, and it was tough. The good news is that it felt like a huge accomplishment, and this morning on my daily commute to work, I improved my average speed by about 1.5 mph and I felt a lot stronger on my bike. That makes me want to go and ride those hills again.