If you can get to the MVT then you will find it very easy fitness wise to get to Nat’l Harbor as there are hardly any hills (with the exception of getting up/down the ramp to the bridge on the MD side). The only potential difficulty you will have is finding your way through Old Towne to pick up the trail again at the bridge. The trail turns into city streets and is not marked. It’s not hard, just a couple of turns but it’s best to familiarize yourself via a map online or on paper.
Go slow over the wooden and concrete bridges, they will be icy at this time of the year.