The motors draw 5A at 12V "at stall" - ie power applied and motor mechanically locked) so they will draw no more than 5A during normal operation.
So, if you work on a 5A continuous draw, then in reality you should require less or even much less. Discharging a battery to 100% capacity is in most cases not good for its cycle lifetime, so designing for less or much less than 100% discharge is usually wise. For first calculations the safety factor built into the "continuous 5A" assumption above gives you some safety margin.
As you note 5A x 1 hour = 5Ah, and 2 motors = 10 AH.
I 12V x 10 Ah battery would allow 1 hour of operation at stall level currents for 1 hour at 100% discharge.
For 12V you could use a 12V SLA (sealed Lead acid) battery, or LiIon or LiFePO4 or NimH or. Initially best cost-performance will probably be obtained using an SLA battery. These will not have the lifetime or peak current capability of some alternatives but are a (relatively) low cost starting point.
So - as a starting point a lead acid 12V, 7Ah "brick" battery as used in many alarm systems and similar, would be a usable starting point. Using eg LiIon or LiFePO4 would give better results but initial cost is much higher.
Once you have 'played with a 12V 7Ah SLA battery you will have a better feel for what works well for you.
Note that though that webpage is mainly about the 30:1 gear ratio motor.
Higher gearing = lower top speed but more torque so better operation in rough conditions and when the battery is getting low (although you should never run the battery so low that it matters). The 5):1 version MAY be a better choice - but at the expense of top speed.
THESE may be their wheels. If so they are 90mm in diameter. On a 50:1 motor you'd get about 180 RPM "wide open" or a speed of about
180 rpm / 60s x 90 mm x 3.14 = 850mm/second ~= 3 km/h.
The 30:1 motor would give about 5 km/h as long as the overall load was not excessive.
At these speeds air drag is minimal and friction and any slope are the main power consumers.