I can identify with the information addict comment- I myself suffer from that, but I try to view it as a good thing, not a bad thing
I've never been on the Lake Mineral Wells Trailway, but it sounds really intriguing, and I'm thinking we might head up there sometime, if we get the chance (it looks like its pretty far from San Antonio, but we could maybe take some leave and head up mid week sometime- who knows).
I would think you'll be fine to do the trailway. From what I read on the park website, and what experience I've had on rec trails that are created from old railroad beds, the 20 miles should be pretty easy- all flat. If you are in doubt, just don't hammer, and enjoy the scenery (and that sounds like it would be hard NOT to do)

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as far as numbness & pain go- I would first recommend that you change your hand positions, stretch your arms, etc periodically while you are riding- to get the blood flowing. Do you wear padded cycling gloves (the 1/2 finger kind, i would assume). What about the padded cycling shorts? Although wearing a giant diaper (or so it sometimes feels) isn't real appealing, that will help make a difference if you haven't already tried it. Otherwise, I would be sure to stretch your legs fairly often- stand up and pedal periodically. When you stop, stretch really well.
I think you'll be fine. 20 miles at a leisurly pace shouldn't be too bad, esp. if you are used to doing 18 or so periodically. Just take it easy, enjoy yourself, and make sure to drink plenty of water when you enjoy your lunch

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it sounds like a great trip, and I am really jealous.
Incidently, we had a rail trail up in Boston, and since we lived next to it, we would ride it several times a week. one thing that I would be careful for are little kids on bikes (with families)- its a great place for them to learn to ride and enjoy it, and not get hit by cars... but they are still learning. and when they panic, they seem inclined to look directly at what they are trying not to hit, which means you, and they will ride right into you... give them a wide berth.