Like qmsdc15 said - it's light enough to ride (and MUCH cooler) pretty early now - take advantage of it. With sunrise at 5:42, you can actually be out a bit earlier and it's quite rideable with minimal lighting. Ride to work if you can as there's benefit in getting your body used to being on the bike a lot. For century prep, you can save your long rides (and you'll want some) for weekends. You want some time getting used to riding in hot weather, too, as September weather can be cool, hot, or in between, or sometimes all three!

Which century? The PPTC Backroads Century (9/19) is somewhat hilly (and they have a great web site up with routes info and training suggestions). The Civil War Century out of Thurmont (9/11) is very hilly. In my experience, you may want to do some riding on hills besides racking up miles on relatively flat trails to get ready. Both rides are very well supported and fun to ride. There are some great hills in Arlington that probably are not too far from the Custis Trail.

re: Lights - you don't really need much if any this time of year, anything bright enough to be seen by car drivers, maybe with a flashing mode, should be OK. For taillights, I like the Planet Bike Superflash or the knock-off Performance sells for lots of light (use strobe setting) in a small not too pricey package that can just stay on the bike. Dinotte headlights and taillights are what I use for dusk/dawn/dark commuting; probably more than you want to spend right now.

re: Commuters on 'closed at dark" trails - LEO can't tell who is and isn't, but someone with helmet, mirror, rack, panniers, multiple lights, reflectors, etc. probably is and may just get a reminder about trail closing, while the folks with no lights, no helmet, and no clue will get more attention. I've heard of examples of this (W&OD out by Rt 28) during fall and spring, never experienced it 1st-hand.