I have an MTB that's not far from that situation. And it's too small for me now that I know what a bike should fit like.
I'd like to get a new one, but.....well.....other things get priority usually for spare funds. For now.
I'm not even sure I want one! I guess could be fun on gated-off logging roads, but no more single-track for me on a MTB... too much risk of head injury.
Sounds like our situation is similar. Short story, bought a Trek late 80's, broke the frame in Moab, and decided to go down a frame size with the warranty replacement, which was nice at the time for some of the stupid downhills we were doing ('nad clearance!) but as you say, now that I've banked a ton of miles on a bike that FITS me, this old thing is basically FUBAR... I think the glaring thing to do first is a seatpost with more setback. I took it for a casual spin this afternoon and could instantly tell, this thing wants to [bleep] up my knees, haha...
HiredGun, yeah, I wrote that on my pedal wrench many years ago! These aren't factory pedals, in fact I put them on there.... in the 90's..... wish I'd put some grease on the threads, though.
I have SPD's on my Roubaix and got MTB shoes with the thought that I'd add SPDs to the MTB too.
Somewhere I have a set of platform pedals with an embedded SPD cleat. I want to put them on so I can use it with regular shoes with the kids or on trails. We have a local Army Corps of Engineers lake with 25 miles or so of trails.
I need to pick something else for pedals and shoes for the Roubaix, though. Haven't really delved into that yet.
Pop-up storms yesterday kept me inside, so I did 20 miles on the trainer. Boring, but useful.
I like the Speedplay pedals a lot. If you want to try a pair to see if you like them, I have a spare set I could send you to try out. You'd still have to get the SP cleats for your shoes of course but at least your cost to just try them would be minimal.
These aren't factory pedals, in fact I put them on there.... in the 90's..... wish I'd put some grease on the threads, though. 
