A nice laid back ride with some technical parts. Not very hilly, but it is gnarly in places. Be prepared to just get off and walk due to some rough spots in the woods. This ride is around 96% ride-able at an intermediate level.
The first part of the ride has you taking the Bruce Freeman trail from end to end, starting in Lowell. This is long, scenic and very laid back. Don’t expect to go fast, and consider stopping to take in scenery. Along the way, there are two ponds you can swim in, one for a fee. Bathrooms are reasonably located in Chelmsford at Hart Pond, Acton at NARA park, and at the train depot cafe in West Concord (also a nice place to get breakfast if you start early). You should fill up on Water in West Concord.
Once you get to the end of the Bruce, you start to mix it up between paved roads and well maintained fire-trails and single-track across several towns. Terrain ranges from nice dusty farm paths to deep woods with rocks and roots.
In Lexington, you ride along Battle Road past major historical landmarks, and eventually you’ll end up in Lexington Center where there’s spots to get coffee, and public bathrooms. Parts of the ride also follow the Bay Circuit hiking trail. You will ride very close to the Old North Bridge, DeCordova, and other touristy spots along the way, so consider doing this in Fall or Spring, or on a weekday. There is a nice seasonal farmstand, where you can get snacks if you are running out in the last half of the ride!