Can bail out at numerous stations on way up or back or start/finish with train either way to Tarrytown and cut distance in half which can make it a C ride.
Use pedestrian path on Broadway Bridge.
Lunch at Tarrytown, half way. (Headless)Horseman Dinner on corner of Rt 9 and 448.
If time is a concern, stay on Riverdale Avenue instead of taking a right onto 252 St. Then follow RDA to the left onto 231 St and then a right onto Broadway, but you will miss some of the nicest riding in The Bronx.