pop off your wheels, and put a millimeter ruler across where the axles go on the front and back of the frame, and measure the inside spacing. it /should/ be 100mm in the front, and 135mm in the back, but disc wheels might be different. whatever wheels you get will need that axle spacing.
if your bike is 700c now, you'll need 700c wheels, period. these are also called 29er in the mountain bike widths. you probably want to get 'trekking' or 'touring' wheels rather than racing/road bike, same thing but a bit heavier duty and wider (racing wheels are ultra-light, and often fragile)
the only other common wheel diameter is 26 inch, used on cruisers and traditional mountain bikes. 650B is a rarer size, and 27" is an old and obsolete size for low to mid range road bikes.
rims are alo referred to by their ETRO size. 700c is 622mm. 26" is 559mm. a tire commonly called 700C x 32 is called 32-622 in ETRO sizing.
for your needs, they have to be disk brake hubs, the axle nut to nut distance is correct, the rim diameter is correct, and the rim's width is suitable for the intended use, you're good to go.
you're probably not going to find a lightweight road bike wheel with disk brakes, disks are mostly used on mountain bikes, and increasingly Cyclecross, but those wheels are optimized for strength, for pounding over rough terrain, hopping rocks and such.
I'd say for your 40 mile ride, try and do a 25 mile ride or two first (if you're not already going that far), and rest the two days before the 40. drink lots and lots of water and nibble on energy bars for the 40.