good routes through Mexico
Adding to Garry's suggestions :
If continuing south from Mazatlan stop in for a look at the little town of Mexcaltitan , only 30km west off the main highway Mex15 north of Tepic at Santiago Ixcuintla. It is on a little island in the mangrove swamps, good for wandering around for an hour or so .Reached by boat only , there is an okay parking area for bikes and cars at the boat landing/ticket office at the end of the road. south of the island .Boat trip is only 10 minutes but interesting to see some of the wildlife. .
Take the free road Mex15 from Tepic to Guadalajara- worth a wander around the old city, then continue on free Mex15 along south shore of Lake Chapala then on to Patzcuaro ,lake and town quite touristy but neat towns nonetheless. Good gs-able gravel road alternate north along the east shore up to Tzintzuntzan and Quiroga Then to Morelia , and thereafter stay on Mex 15 east for some really neat mountain scenery and great road to Zitacuaro, Toluca and Mexico DF
So many great roads to wander around on. If this is your first such trip you really can't go wrong, every road is new so don't sweat it if you are limiting yourself to pavement and well used gravel routes. Just take it calmly and enjoy.