Thanks for the advice, everyone!
I think my first step will be to decide if I can/want to spend the money on a new pair of shoes, and have one for spin and one for the bike. If I do, then I will have to decide on all the pedal systems. But at least now if I don't, I won't feel too bad about sticking with SPDs. I think I am just not used to them on the spin bikes yet...
I think they are...my shoes are a 38 and there isn't a lot of room to move the cleats further back. I do keep riding after the pain begins, but then my foot is very sore when I try to walk for several days after. Thanks for the advise, though. I salesperson at REI told me he thought I maybe a larger surface cleat area might be the answer, but it is very expensive to "guess" this may be my solution. My shimanos are wearing out, however, so a new pair of shoes are needed. They will still be good for my commute, but not longer rides.
If it's sore days later, it doesn't sound like your typical "hot foot" pain. Sounds more like plantar fasciitis or something like that. I had some bad pain out at the 5th metatarsal about a year ago. Moving my cleats helped a lot, but it took time. Turns out I had given myself peroneal tendonitis that I had to go to PT to rehab. The original cause was from some pelvic / hip issues. Hurt more when walking, but I was tweaking it on the bike. I also was just spinning then with no-float SPDs. Don't know if it was the lack of float or the size of the cleat (they were too far forward too). But I don't aggravate it or make it flare back up at all with my Keos.