Hi jimbear.
Welcome to our Mattress Forum. 
It sounds like you’re living a pretty active life despite your retirement.
Is the back pain worsened when sleeping on your current mattress? You may find this thread interesting as it gives a breakdown of the most common reasons for back pain and offers some solutions when it comes to mattresses.
If the foundation is in good shape (no bending, etc) and is adequate for the mattress you buy, then it’s smart!
Depends on what the cause of the back pain is. If it’s a lack of adequate support for your sleeping position, it may not necessarily be that you need a firmer mattress, rather adequately thick comfort layers (in the right firmness, whatever that may be for you). You can find the recommendations we make based on sleeping position here.
Flippable mattresses are fine, but any mattress made with adequately durable materials is going to have a long lifespan. Some latex mattresses ‘live’ up to twenty years!
Are there any mattresses you’re looking at in particular?
NikkiTMU