Wiggle

On August 10th I am hoping to ride in a local metric century (100 km). I've gotten back into cycling this last month and have been using a road bike. Is there any particular method I should use to prepare for this? I'm in generally good shape, I favor weightlifting more than cardio but I'm capable of decent endurance compared to the average couch potato. I've been putting in time on both the roads and trainer. I can do shorter rides at a decent pace, example a 25 km round trip in just under an hour with just a 30 second break for water other than the quick breaks at stop lights and some light coasting. I've also done longer rides of 40-50 km round trip with a couple breaks of 5 mins or so. I am somewhat heavy for a cyclist (188lbs currently) but am shedding a little weight and expect to be in the low 180s by the time the ride happens.

Are there certain benchmarks I should be trying to hit over the next two months? Is there an ideal length of ride to help me reach this level of endurance? Should I avoid challenging rides for a couple days before the big ride?

Carbonfiberboy 

Track your weekly mileage (or kilometerage). If you can ride 150 km a week for at least a couple weeks before your century, you won't have any trouble. Ideally, you'd also gradually increase the length of one ride on the weekend. Riding up every hill you see is a good practice.

Yes, you should ride easier that last week, tapering down to nothing on the day before your event.

Machka 

If you're comfortably doing 40-50 km rides now, this weekend, go out and do another.
Next weekend go for 60 km.
If you felt really good on the 60 km, next weekend, do it again. Then the following weekend, try a 65 or 70 km ride.
If you kind of slogged through it, next weekend, do a 40-50 km ride. Then the following weekend, try 60 again.

If you feel really good with a distance, repeat it the next weekend, and then increase it the following weekend.
If you slog through a distance, go back to the 40-50 km ride the next weekend, and then try it again the following weekend.

Do you know what terrain the 100 km ride is? If so, choose routes that include that terrain.

BKSams

Until this year, the longest ride I went on was 15 miles, I turned 50 on the 5th of April this year and did 30 miles in 1h:34m just to see if I could do it. The same ride now takes me 1h:10m, my last ride was 88 miles, what I do is to set a certain mileage and just do it, I just flat out refuse to get off the bike until I hit my mark.
April-30mi, May-50mi, June-72mi, July-88mi, August-100+ I'm also on Strava check it out.

rm -rf

You could probably finish the ride right now. Machka's advice is just right. Do some 60 km or 70 km rides without feeling wiped out and you'll handle a 100 km pretty well.

Is this a big group ride? That always makes the distance easier for me--it's motivation to keep riding and the miles/kms go fast. But don't start too fast by trying to keep up with faster riders. With more experience on longer distance rides, you'll have a good idea of how fast you can go.

Wiggle

Thanks everyone, the ride went well Just a couple things I noticed if anyone else is in a similar position:

I felt very fresh under 50km and had a tendency to sprint up hills to get them done quick, which worked great in terms of holding a good pace but I think it sapped my gas tank a bit much for later.

Luckily, from 50-75km we had flats with a headwind but kept our group of 5 riders in a tight paceline so was able to hold a solid speed without sapping too much more energy for the last 25km (which had alot of hills).

Comfort was pretty good, got a little seat butt but that's about it, feel fine today. I think next year I'd like to attempt the full 100 mile course.

I didn't think I'd like riding in a group, I tend to like doing exercise solo but it was actually pretty great. The drafting advantage was nice and the ability to talk with the other riders helped pass the time without focusing on how tired I felt. We had one strong rider who helped pull us and then the rest of us were about the same level of fitness.