Translation:You should exercise more.
I think it represents the order of two events.
したらいいです(past->now) The speaker concludes that based on their experience that it worked for them.
するといいでしょう(now->future) The speaker assumes that based on shared knowledge.
If you are talking with your close friends, the former one is more likely to be accepted as an advice because your experience is reliable.
It also hints that it will take much time to get a good outcome. If you think it won't, you can simply say するといいです(now->now).
Yes, actually ほうがいい works with both present and past forms are both grammatically correct and have both the same meaning more or less. I feel like the construction with the past form is more common, but I would not bet on that. To make it into a past scenario, you use the past form of いい in the construction, よかった so it becomes ほうがよかった
The expression ... ほうがいい is actually a comparison. A よりB のほうがいいです (B is better than A). It can be preceded directly by a verb in the plain present or past tense, a noun or pronoun plus の, or a demonstrative ending in の (その). The adjective いい could be replaced by others. そのほうがおいしい / おおきい / たかい です. That is tastier / bigger / higher or more expensive.
When ほうがいい follows a verb, it's used as a kind of recommendation or advice about what to do. You (had) better ... / You should ...: べんきょう (れんしゅう) したほうがいいです. You should study / practice. As to した vs. する, consider that in English we can say: It would be better if you exercise(d). Recommendations can be seen as not just advice to DO something, but as exhortations to get something DONE. The latter is probably a little more emphatic.
Website to help you under ほうがいい better. http://wordgrammar.net/jlpt-n5-grammar-%E3%81%BB%E3%81%86%E3%81%8C%E3%81%84%E3%81%84-hou-ga-ii-1-explanation-and-examples/