Are there ways to keep long-distance land transport costs down in France when not planning in advance?

Ten years ago I travelled around Europe for the first time and in some countries I was very frustrated by high cost of trains and lack of alternatives to them.

I remember being in Cannes with a friend who had tons of money. I had already spent most of my inheritance despite trying to keep costs as low as possible.

We wanted to do something vaguely like Cannes - Avignon - Lyon - Dijon - Paris (I can't remember exactly any more).

But it seemed that each hop in back-then prices on the TGV was about 50 euro whereas we could go straight to Paris for something like 100 euro.

Furthermore, unlike Germany which I was used to travelling around in by then, there seemed to be no cheaper options for slower trains. I always chose the slowest train because I was never in a hurry and enjoyed the scenery. But in France there was no such option.

In other countries I knew how to travel in, like Australia or Canada or Mexico or would take a long distance bus. But as covered by another question, there don't seem to be any long distance buses in France.

So this lone option of the TGV made it at least double the price and I had already blown some money on trains from Monaco to Nice to Cannes.

We argued and debated and decided it would be worth the gamble to take a train to Marseilles where we expected we would have a lot more options than in Cannes. But when we got there we found it bigger and busier but with the same expensive TGVs and still no other cheaper slower options.

We gave up and just went straight to Paris very grumpy. To this day I have been put off trying to travel around France for the same reason.

So enough whiny anecdotes and on with the actual question: Were we doing it wrong? Are there ways to visit all these places without blowing the budget on TGV tickets? Is there any other mode of long-distance overland travel within France? (No flying please, I want to go on roads or tracks through the countryside.)

Oh and just to make it difficult, I like to travel with no plans. So I can't buy passes or book tickets months ahead and I'm too old for student specials.