How do you guys maintain a healthy weight?

I'm 178 cm and my usual weight is somewhere between 12 and 12.5 stone (around 170 lbs), even though my weekly mileage is currently only at about 30 miles I am struggling to keep weight on, I've already dropped below 12 stone and I have no idea how much weight I'll lose as my mileage increases. I am trying to eat as much as possible but feel like I'll be sick if I eat any more. The same thing happens to me every year.