I did a spot-on 60 mile long suburb-to-woah-where-the-hell-am-I-rural-place to city to suburb fixed today, I have tried it 3 or 4 times with mixed results (ridiculous humidity leading to dehydration, walking up 1 or 2 hills, getting lost, etc) but I nailed it today. I did it in just about 4 hours giving a spot on 15 mph average. Headed out to the rural place is a literal climb, so I'd say I only managed 12 mph average at best. The worst hill (Waterloo Rd in Devon, for any people in the PA Main Line) was about 10 percent. The rural area to the city is a straight, level bike path. I averaged 18 mph with 3 24 mph sprints.
My point is doing long rides fixed with the right gear (saddle/hip bag, at least 1 bottle, a good route with multiple libraries/supermarkets/etc for water refills) is key to working up to a century and staying healthy in this almost-August heat.
Don't think I'll be doing a FG century on a very tight Track Pro very soon, at least not in the heat. Today broke me down pretty well and it will take probably a dozen more rides until I feel comfortable with my planned century out to Lancaster. My ultimate goal is to do the Philly to Pittsburgh "race" next year.