If you have 36V battery then that is fine!
Long Haul Trucker is the LHT by Surly
If 35kph is your goal, then going with a dependable direct drive hub motor would be good.
We need more information though.
Do you want to have pedal assist or throttle only?
How much pedaling do you want to do?
Is it for commuting, pleasure or physical activity?
22 miles (32km) route with flat terrain and a 3% grade somewhere in your route.
Lets do some keyboard math because I am bored.
You need more battery then just 32km, so lets round it up to 50km. Shallower discharges = longer battery life!
Motor Type Rough energy usage
Minimal Assist (using motor only on hills, slower ~30kph setup) 6-8 Wh/km
Typical Assist (~40 kph with pedaling, motor on all the time) 9-12 Wh/km
Power Hungry (either no pedaling, or hauling a load, or going really fast) 14-20 Wh/km
Lets use 15Kh/km x 50 = 750Wh
750Wh divided by 36V = 20Ah to go 50km on 36V
You got 12 and 15Ah. that is 37Ah. Obviously!
But are they new, newer, old? All affects distance!
Another scenerio, 36V x 15Ah = 540Wh divided by 15Wh/km = 36km your distance is 32km. And your 12Ah is 28.8km.
Remember to store your batteries at 37V, I hope you've been doing that!
Recap, when new your batteries could do...
36V 12Ah = 29km
36V 15Ah = 36km
No need to buy a new battery, obviously. But do your battery packs have a BMS in them?
I say install your 15Ah pack on the bike or however you want to do it. Take care of it.
This is what I'd do, depending on the size of the pack. Place both batteries in parallel and have one 36V pack of 27Ah. Place it in the triangle of the frame, this will net you 65km, and you would only use half of it, making for a shallow discharge of the pack which is better then going all the way down to the LVC of the pack of 30V or 31V. Something to consider also, is maybe charge the pack only to 40V, instead of 42V which is the max. Again, longevity of the pack, want it to last forever! Obviously, but it will degrade over time, number of discharges, charges, depth of discharge etc etc etc.