I guess from Avila northbound you have also plenty of other options to get to the Camino de Madrid.
Using the local and provincial roads, you can easily connect your Caminos between Medina del Campo (on the Levante) and Valdestillas in one day walking (about 27 kms; it could be a bit shorter if you find a more direct way to walk than the provincial roads); another option could be going from Arevalo to Coca (or to Nava de la Asunciòn) , passing by Santiuste de San Juan Bautista: again, 25/27 kms.
As you have certainly noticed while you were walking your Camino de Madrid, in that countryside provincial roads have a very little traffic of cars, so one day walking along them can be acceptable, even if on tarmac.
Whenever you get to Medina del Campo along the Camino del Levante you could also go on along the Camino del Sureste for one more stage, till Tordesillas, and then try to get to Simancas: it's even shorter than the previous options but maybe the roads are more crowded in that area; there could be some itinerary along the river Duero but some more accurate search has to be done in order to find a pleasant and safe track to walk!
Anyway, the Camino del Levante/del Sureste (stint from Arevalo to Villalpando) runs in parallel to the Camino de Madrid (stint Santa Maria La Real de Nieva to Medina de Rioseco) for a very long distance and there are always 23/28 kms walking from one track to the other: so you can find plenty of opportunities to switch from Levante/Surest to the Camino de Madrid in just one day.
Managing properly the map at the following address you can plan several options for your "Camino switch":
http://www.caminosantiago.org/cpperegrino/caminos/nuestroscaminos.asp