Speculation of course:
The first few iterations must have been very different. I picture that it took several (many thousands?) of blind alleys where he got killed before finding the wall but still managed to get back and rerun the transporter. After all, he could not have had any of the hints the first time.
At some point, maybe early on, I suspect he just left his clothing behind to dry and never went back to pick them up. I doubt that he would have any qualms about facing an enemy unclothed ("silly human prudishness"). After that, every subsequent iteration had the previous Doctor's clothing waiting for the next one.
With enough iterations, eventually all the paths converged into a stable loop (an attractor, in Chaos Theory language), which is what we see. The whole thing seems to take about a day, so he literally had as many as 2.5 million tries before reaching a stable loop.
About the age of the painting, note that we only see it the first time, never in subsequent loops. We don't know whether it's there for millions of years. I suspect the painting ages and its eventually gone. I suspect that on one iteration he gave himself a lot of time to figure things out and he did the painting himself (there is no evidence that all the initial iterations took merely a day, we could have had weeks for some of them), or maybe he started it in one iteration and then spent several iterations completing it. Eventually it was "done" and the "painting exercise" stopped being part of the iterations and the painting became an item to observe. After tens of thousands of years it becomes an irrelevant rag - a small inconsequential variation of the stable loop.
We only see the stable loops and have to guess at what happened before.