First Time MTB'er trying to get fit

Philgr

Im a first time biker trying to get fit.

Im 37yrs old, 5ft 9'' and 13 stone. mainly carrying all my extra weight on my belly, chest and face. i really want to get a min of a stone off ASAP and get my fitness back.

I have got the bike, i have a short route from my house, 4 miles total with a good mix of road, off road and a 1 mile climb, and once i get some fitness back i will venture out onto the trails in the lakes

I have got a bike computer with a HR monitor on, my current resting heart rate is 67 and on the last couple of runs out my max recorded HR was 182

My Monitor will measure 5 diff zones that are currently set as follows:
L1 30-119
L2 120-140
L3 141-159
L4 160-180
L5 181-230

Can anyone please advice on the correct setting for these 'Zones' for me, i am going to record them for all along with the other info for my rides out to see if i can see any improvements

Thanks in advance

Phil G