For distance u might need more.juice than tony i think.
I dunno for hills but going at 800w constant (versus 500w i usually consume.on flats)
U might.need 400 wh (more or less) for 30mins riding.
This depending at what speed u go might bring you either 10km or 15km.
General rule of.thumb for mountainboard energy consumtion is 15 - 20wh/km (except for some builds like @SimosMCmuffin who got something like 11-12wh/km I think )
So… for flats only 13 km (8miles) might take = 13km x 15 - 20 = 195 wh - 260 wh
Tonys battery = 4.9ah (margin) x 37v = 181,3 wh
So i would say u might need 8000mah at least more or less
Ok didnt get to finish but I wanted to point out that if you would find out average watthour rating you might consume, then you could tell would your battery would be enough for range you desire.
Taking, 25wh / km, you might need 325wh of energy, more or less.
the 8ah pack (7.6ah - with small margin), at 37v = 281.2 Wh
So it looks like 10 ah pack would be more suited:
~9ah (with some reserve), x 37v = 333Wh
This means, that in theory, you could ride at 333W of power for an hour, though im not sure you would get such average number… but who knows
My estimate is that you could see something more like 600W average at least (dual motors, too), so taking 666W (hah good number of choice, right), 30minutes or ride should be possible but speed is hard to tell at which you would go at such power level and also versus the hills/downhills you take.
Regen might also give some energy back but I dont have a good estimate for that.
Non the less, it looks like if you want some room for your battery, it would be best to go with 10s 10Ah or maybe even 12ah.
This comes from personal opinion and some numbers I crunched it. It might be possible to go the distance with 8ah pack but I wouldnt bet on that much, as it would produce close to 285 wh, if you calculate some reserve into it.
And 285wh at 20wh/km, comes at around 14km (so only 1km reserve), so it is hard for me to speculate would you come below 20wh/km mark or not with the extra terrain conditions you mentioned… though then again i havent done much data on hills, and this might be all false.
Though I hope i gave you some impression to go with and @aigenic also explained why it would be better to go with a pack with more power for example.