Just started an exercise routine - How much to eat?

Hi guys, I have used MFP and exercised on and off for a good few years now! I got really into the gym last May/June, but had to cut back a lot due to diagnosis with endometriosis and a following laparoscopy. Anyway I felt recovered on 1st Jan and this seemed like a convenient time to try and get back into my routine.

I now exercise about 5 days a week (although I do try to walk on rest-days) and count one day of exercises as a long walk. So 4 days in the gym total. I have recently been trying to divide up my days into 1 leg day 1 arm day 1 run and ab day and 1 full-body or cardio day depending on how I feel. I always start my workouts by running for at least 10 minutes and spend 45mins to 1hour in the gym burning about 350-500 calories.

At the beginning of the month I was only eating about 1250-1400 calories per day except when I couldn't count calories if I went out on the odd day. I'm 26, 5'10 and 132lbs so I don't really need to lose weight - my goal is really to lose fat and gain muscle. My fitbit reckons that if I do the bare minimum I burn about 1,750 calories and my Tanita scales suggest my BMR is 1360-1400 (it fluctuates). I try to burn at least 2,000 per day even on rest days.

The past few days this week I have noticed that I have become completely ravenous and so upped my calorie intake a little (I've probably eaten 1550 cals each day I exercised this week). I know it's probably not enough, but I also don't want to go overboard, however am becomming concerned that I do need to eat more.

Can someone suggest a good calorie goal to be working to (does it need to be different on rest days?) and also the best things to eat bearing in mind I'm pescatarian. I'd also appreciate any comments on the routine I'm trying to stick to...?

I am sad because I felt so energised the first 3 weeks or so of working out, but I'm now becomming quite exhausted and it's demotivating (although I do really enjoy my workouts when I go).