Those two sentences are grammatically correct, but they give less information. 'Ten years ago, I could run around the lake within half an hour ' This is simply a statement about the past. It says nothing about the present time. 'If it had been ten years ago , I could have run around the lake within half an hour.' and 'Ten years ago , I could have run around the lake within half an hour.' mean more or less the same. The implication is that they can't run so fast now. The only difference is the phrase 'If it had been', which suggests that maybe the speaker has just tried to run round the lake and it took them much longer.

All three of those sentences are correct. It sounds more casual/ natural to say "in less than" rather than "within", but within is correct as well. The difference between the sentences: "If it had been" - It gives a nuance that you have never run around/ seen this lake before. "I could run" - Ten years ago you ran around the lake and it took less than 30 minutes. "I could have run" - Very ambiguous. You definitely did not run around the lake 10 years ago. If you had you would not use "have"

The revisions are correct! Remember that you don't put a space before a comma, only after.

My try: Back to ten years ago, I would be able to run around/circle the lake in half an hour.\ Were it ten years ago, I might ...