After running a report on Garmin Connect, I saw what I suspected was the case with my training this year, very little elevation. Granted, I ride in very flat areas, SE WI, NE IL, but I'm about 25k in elevation short from last year. I'm going to be doing a fairly hilly, for me, century in 3.5 weeks. There are some short 6-8% climbs around here that I can train on. How many hill repeats do I dare try to cram in during the next 3 weeks? 49 year old male, about 2,200 miles so far this year. I'm not worried about the distance as much as the distance+climbing. Thanks.

You've left it pretty late. How many hill repeats you can stand is in part a function of how fit you are already, and how much mileage you think you need to put in on top of the repeats. Only you know the answers to those questions.

If it were me, I'd probably go for hill repeats a couple of times a week for the next three weeks, interspersed with longer, less intense rides. That'll give you a few days at the end to make sure you've recovered properly before the event.

For your muscles there is very little difference between riding on a flat and climbing. That means 200W on a flat is the same as 200W on a hill. The reason why people say hills are hard is because they will suddenly start increasing their effort on a hill, so if they do 200W on a flat, they will suddenly start pushing 300W on a hill, blow up and start saying that they are not strong enough for that hill.

There are several reasons why people start increasing the effort on a hill:

1) I ride with a power meter and noticed that on recovery rides going at something like 180W and trying to go at a steady level, as soon as I would go up a hill the power went up to 250-280W. It is just so easy to increase power unnoticed when climbing.

2) In order not to tip over, if gearing is not low enough, the minimum speed that you must go at a cadence of 60-70 RPM could put you above your comfort zone and make you blow up.

3) On a hill there is no coasting. If you are used to pedaling hard for a few seconds and then coast a few seconds to recover, that ain't gonna work on a hill, so don't do it on the flats either.

The above can be summarized as "hills are all about gearing and pacing". If you understand that, there is no reason why you couldn't train only on flats and then go and do the hilly century ride and succeed.

For many people hill training is actually just a lesson in going slower.