Alewife has long been known to area cyclists as the start of the Minuteman, which (with the Reformatory Branch Trail) takes you all the way to Concord.  It is also now an endpoint for the Somerville Community Path, which takes you all the way downtown now (since the Green line extension).  And it is an endpoint for the Mystic River trail system.  While this system is less continuous, the new addition of the Clippership Connector makes it possible now to cycle from Alewife to the start of the Northern Strand Community Trail (with a couple of short on-road or on-sidewalk sections), which you can take all the way to Lynn (and to Nahant and the ocean after some current construction is completed).  And it is going to be an end point of the Mass Central Rail Trail, which will (eventually) take you most of the way across the state, and right now mostly gets you to one end of the Bruce Freeman Rail Trail that can get you almost to Lowell.  Of course, there are still gaps in MCRT, with Belmont being the largest holdout, but they should begin phase 1 construction this fall.  (Unfortunately phase 2 hasn't even been designed yet).  And with some sidewalk riding you can get from Alewife to Fresh Pond which connects you to the Watertown Cambridge Greenway, which then connects to Charles River Bike Path aka the Blue Heron Trail, which can take you to Waltham, and to downtown.

Of course we then run into the hard-and-fast metro Boston rule: Thou Shalt Never Cross the Charles.  And I'm less familiar with cycling infrastructure south of the Charles, but on the maps, it isn't forming a significant connected network of trails in the way that things are shaping up on my side of the Charles.

But at any rate, that's five significant trails all landing in Alewife, with further connections to other major trails.

Alewife becoming a major separated facilities cycling hub for Northern metro Boston.