It all depends on how you want to travel and how much time you have to spend.
The toll roads in Mexico are like the interstate highways in the USA. Great for going from point "A" to point "B" but you really don't see much.
The free (libre) roads are like (sort of any way) Route 66 in the US. You don't make much time but sure see a lot more of the country. A few hundred miles a day is a good day.
Mix it up, use both in a meaningful way. research areas that are interesting and plan a route but do not over estimate how far you will drive in a day. Even with the toll roads, with the toll booths and all, it cuts deep into the miles per day.
The best investment you can make is buying Mike and Terri Church's book on camping in Mexico. it will pay for it self twice the first day on the road.