47, out of shape, and over weight. What gear set up would be the best to start with on a single speed bike ?

I started with an 80's Peugeot PFN10 12 speed, so it was an easy matter of finding a comfortable gear combination I could use for the entire daily commute. I ended with a 42/16, not much of a challenge, but quick in traffic, and low enough for some hills. I shifted the 42 to the outside of the spider with an extra set of ground-down bolts, and stripped all the other gears off the freewheel except the 16, which fits against the step on the freewheel housing. Alignment is very close to perfect. Added a single speed chain. Total swap, about 3 hours. A delightful ride.

I started with a 42/16 as well. I'm 44, making progress shape wise. Hey, there's people at work who at least look like they might be in better shape who wonder how the hell I ride 30 miles to work. pici's comment that the 42/16 is quick in traffic is actually very important. If you're commuting you need something you can repeatedly haul up to speed from red lights and stop signs (I hope you're stopping at stop signs and signals - especially in front of motorists - and helping our rep). I have a couple significant, extended climbs on my commute and the 42/16 is the tallest gear I can use right now.
DanO

I'm running a 52/20 because those were the parts I had on hand. 52/20 is virtually identical to 42/16, and I find it perfect for a slightly hilly commute. I just setup the other side of the flip-flop hub with a 20t track cog to try fixed versus SS on the commute.