As above you have the glen loin loop which can be extended with an out and back up the cobbler path for that big mountain feeling. There is a nice wee bit of singletrack through the trees from the fire road down to succoth which is not official and easy to miss but if you look it up it will be easy to find.
There is also the 20 something mile ardgartan peninsula loop which is mostly fire roads along loch long with a singletrack strech over to loch goilhead and fire road back via the rest and be thankful. You have the west highland way running along loch lomond, do as much as you want.
Conic hill as nobeer described is a good run but can be very heavy traffic with walkers if you go back down via the main steps/route. On the descent back you can go right down the grass ridge which will drop you out on the tarmac road going from balmaha to rowardennan, you can cross a wee field here to reach the west highland way and extened the ride or back to balamaha.
Glen loin loop and cobbler path would be my choice, especially on a good day.
https://www.walkhighlands.co.uk/lochlomond/glen-loin.shtml
https://www.walkhighlands.co.uk/lochlomond/the-cobbler.shtml
The path up the cobbler is currently being extended/improved and looks like there will be new paths going up beinn ime and beinn narnain, just go up the patch as far as you like (usually to end of stage 3 in walk description), turn round and blast back down again.