If you start your I-70 journey at the eastern terminus, the first mandatory stop is Patapsco Valley State Park. The park spans 32 miles along the river valley and has two primary biking areas. Known as Avalon and McKeldin, they are separated by less-developed areas of the park and each have their own network of trails that can keep you busy for a few hours. Most would argue the Avalon area is the #1 mountain bike destination in the Baltimore region.
In 2002 a group of us struck out on what we hoped would be a ground-breaking trip, one that would connect the Avalon and McKeldin riding areas by as much singletrack and dirt riding as possible. With prior field research (i.e. lots of riding) and a new park map that had just been published showing a huge network of "unmaintained trails," we were successful in joining the two areas together for what was later dubbed the Tour de Patapsco route, a 50-mile round trip connecting the two local jewels. Now the park management itself is working on making this a legitimate end-to-end trail dubbed the "Thru Trail" complete with sweet bench cut singletrack where there was once barely-existing slivers of eroded dirt. The "Thru Trail" is not complete yet but a lot of progress has been made. Anyone with a sense of adventure can purchase a topo map from the park, which touts "over 170 miles of trails." Then just pack a lunch and build your own adventure.
Patapsco itself is a huge success story. In the mid-90s some trails were in bad shape and the finger was being pointed at the two-wheeled trail users. Threats of closure were in the air. Mountain bikers rallied, fought to keep access and became a tour de force in trail advocacy and maintenance in the park. Local trail advocates have a great relationship with the park and a huge influence on trail projects. Just as important, the park staff understands and embraces IMBA trail building standards and has seen huge success in building and maintaining sustainable singletrack.
Don't think you are done with Maryland yet, the I-70 fun has just started. Just west of the city of Frederick, the Catoctin ridge forms the easternmost portion of the Appalachian mountain range, the oldest mountains in the world. If this isn't enough to get the a mountain biker salivating, then consider that just a few minutes off of the highway there is a trifecta of mountain bike trails, Gambrill State Park, the Frederick Watershed and Greenbrier State Park, that make this area worth a stop.
For a stranger in a strange land Gambrill State Park is a best bet with its well-marked trails and kiosk map to keep you from getting turned around. The yellow loop, maintained by local trail advocacy group M.O.R.E. (the Mid-Atlantic Off Road Enthusiasts,) will raise your heart rate to the max on some of the steep climbs and keep you on your toes with rock-filled descents. Bonus points for this being one of the most rain-resistant trails in the area due to its high rock content.
The blue trail offers even steeper terrain and will take you into one of the most beautiful valleys in the park. The price you pay on the climb out of the valley is worth the views though. It's a linear trail that will lead you into the adjoining Frederick Watershed where there are many more trails (but generally unmarked) and much more terrain. Don't expect to find the good stuff without some local knowledge though. Keep on the lookout for bears, rattlesnakes and hunters (in season). And pack everything you need including plenty of water since there is nowhere to resupply. Spare tubes are a must as the rocks in the area love to eat tires and tubes for lunch. This area is for the more adventurous: unmarked trails, easy to get lost and little traffic to find you.
Just up the road is Greenbrier State Park which hosts an annual spring race that serves as the Maryland State Championship as well as an Olympic Qualifier points race. The trails here are on the wider side but have plenty of rocks to keep you honest, climbs to keep you working and downhills that you can rip. Finish off the day with a swim in their big lake before you hop back in the car and swing back onto I-70.
Maps for Patapsco Valley State Park, Gambrill State Park and Greenbrier State Park can be purchased online at the Maryland DNR web site.