The GAP is doable by just about everyone, we did it on a tandem this past summer, here's our crazy guy journal:
http://www.crazyguyonabike.com/doc/12687 . As other have said the whole GAP is the thing to do, but if you don't mind back tracking you could park in Cumberland and take the tourist train up to Frostberg and then from there ride to Meyersdale to overnight. From Meyersdale you can continue north for part of the day (Rockport?) then turn around and head back to Cumberland. The extra distance on the second day would be mitigated by the 20 mile down hill ride back to Cumberland. This brings you through most of the GAPs big engineering features (tunnels & viaducts).