I've done the DC round trip once. I tried a year before that and didn't get all the way to DC (but I went and returned as far as I did by bike. Didn't suffer the ignomy returning on some other transportation.)
The time I tried and failed, the longest training ride I did was about 45 miles. My plans were to take 5 days down, one day of rest and 5 days back. That is about 67 miles a day
Wouldn't you know? I averaged just about 45 miles a day! Isn't THAT a coincidence?
There were days I pushed myself - up to 72 miles one day, for example. I had to push myself to go 30 miles the next day. At the end of the trip, I was really feeling progressive exhaustion.
You might want to grit your teeth and push through for a 2-day trip. For a 10-day trip, it might be possible.
This year, I was successful. I planned for 7 days down and 7 days back, two days rest in DC. About 48 miles a day.
The longest training ride I did the sucessful year was Oscar Swan's Potato Garden Run Ride. With getting lost near Alliquippa, that was over 70 miles. One weekend I did a distance training ride both Saturday and Sunday.
Even so I wish I had trained more. It would have been easier and more fun on my trip. As it is, I felt good on the trip, but was wasted for a week when I got back.
If I were to do it over again, I would train much harder - do a century, maybe do the MS 150. For the round trip, I'd do 6 days down, 3 days rest, 6 days back and another day rest. 8 weeks training with one century and a 2-day 150 ride would be just about right for 56 miles a day.
I think if I were more or less fit and didn't train much on a bike, then about 30 miles a day will be way on the far edge of what I would want to do. I've heard some experienced folks, much younger than me, agree with that.
Of course, "more or less fit" is different, say if you're 23 and ran track in college compared with a couch potato. I'm in my 50's.
There is a trail book that I found kind of helpful, but kind iof commercial and an irritant, too.
http://www.amazon.com/Trail-Book-2009-10-Jeff-OBrien/dp/0979210828
It has a vinyl trail map that has lasted in my sweaty back pocket for roughly 1000 miles. The roads off-trail that the map shows are helpful - even when you also have a road map or two.
But, for example, the book doesn't show taht there is lodging in Paw Paw, WV, because no one from there advertised. Paw Paw is a major stop over.
The other books I bought are more sincere, but were less useful. That included "Linking up" and the "Trail Companion"
http://thelocalhistorycompany.com/books/097118352x/pages/097118352x.html
The Trail Companion, is good to read for the history of the rails, but not as useful for traveling. Linking Up is geared towards riders that are supported by (make me gag) internal combustion machines and such.
The history of the rails in brief:
"There were some people who started a railway project to compete with the Pennsylvania RR. They made some foolish decisions and were (unexpectedly) sandbagged by people paid off by Pennnsylvania Railroad. Formerly wealthy men were ruined. The rails were eventually purchased at rock bottom price by (guess?) Pennsylvania Railroad."
Repeat a half dozen times until a bike trail forms in the remnants.
There are ways to bipass parts of the GAP trail to Cumberland, I guess, but the trail always seemed fine to me.
Not so the C&O.
It was originally conceived as a hiking trail and there are people strongly committed to historic accuracy and non-development.
On the C&O, when it rains, it becomes apparent that you are riding on century-old mule shit. When the rain stops, you will learn why they use dead horses to make glue.
Thre are parts of the C&O where it would be hard to avoid the trial - particularly the first few miles out of Cumberland, with 1000+ ft climbs. Other parts, it is pretty good to take the roads.
Around Hancock, there is 20 miles of paved rail-trail that is parallel to the C&O and often in site of it.
From Williamsport through the Antietam Battlefield, Sharpsburg, and to Antietam Creek is a lovely ride on the roads.
There is a ferry ($1 American) over the Potamac near Leesburg, VA, then a paved rail trail from Leesburg to DC.
All in all I've done about 600 miles "on" the C&O, but that was really about 370 on the trail and 230 on roads. I don't regret that one bit. There is about 20 miles of the C&O that I missed -both trips, both directions.
Maybe heaven is on that 20 miles, I dunno. Doubt it.
There seem to be plenty of places to stop for food, water, beer, or beds on the GAP, but far fewer on the C&O. Plenty of places to camp on the C&O, but if you are doing B&B's and motels, you have to plan.
If you want, you can message me a month or so before you leave. You can buy me a beer and I'll go over the specifics using maps, diaries, visual aids, etc. I'll blither on until you start napping -and maybe past!.
Mick