When you are not used to riding then you tend to ‘burn’ off lots of water.
Your circulation has not adapted to the new regime, which means that instead of an increased blood supply to the skin to cool off, you end up sweating more, plus drinking water of itself has a cooling effect.
Keep your hair short, but not too much or the sun may get to your scalp.
Check out your clothing, likely it will be retaining too much heat, there are modern fabrics that will help disperse it.
If you sweat heavily then your groin may become sore, and may lead to boils, so temperature control is important.
I did notice that you said you were ‘building up your leg muscles’ and I take that to mean that you ride in fairly high gears.This is a common mistake that new riders make, you will tire faster doing this, my advice is to pick a gear which you can turn at around 60rpm, changing to a lower one if you are forced by the terrain to pedal slower.
You can get cycle computors that display your speed and pedalling rate(cadence), keep your cadence above 60, the optimum for time triallists is around 80 rpm though really good riders can get to around 110rpm.
As you ride more you will find that these things seem to sort themselves out.
Distance and speed are less important than regular rides, something like three rides a week should be enough, one perhaps being a good deal longer then the others, maybe take a break at a cafe or something.
I’m getting back into riding after a very lazy number of years and so I’m only doing around 100 miles per week.
The most I’ve done in 12 hours is 219 miles, but then that was a race, I have ridden 260 miles in 24 hours in what we would call a reliability ride, the continental chappies and chappesses would call them randonneur events(Coldy will know what I mean).
During the racing season I’d maybe do around 350 to 400 miles per week dropping to around 240 miles during the off season, unless I was resting up for a big event in which case I’d drop my miles by at least half.
Most in a week, I did a tour of the Yorkshire East Coast, up to the Lake district(Cumbria), and back down through the Yorkshire dales, that was around 900 miles in a week, with full camping kit on.That might seem like a lot but in summer you have up to 17 or 18 hours usable daylight so it isn’t as tough as might seem.