I regularly walk the stretch of the via Flaminia from Acquasparta in Umbria up to the Ponte Fonnaia near Massa Montana. It's only about 3 miles each way but the walk is parallel to the Martani mountains which are beautiful. The walk does come to an abrupt halt where the river Naia crosses it as there is no bridge unfortunately. Walking boots would probably cope but far too wet for trainers!
Careful, if you use this road off from the Flaminia, watch out for the Nigerian prostitutes that might jump on your hood. Badly kept, lack of coherant road signs....