Please help, I'm so confused!

Hi, firstly can I say, I've never done posts before, so please bare with me!

Ok, I'm a 48 yr old 5/5 female, weighing 152 lbs, who is very luck not to have to work, so up until recently I've had a very lazy lifestyle! I decided I wanted to lose a stone, tone up and just get a bit fitter, so I got a trainer and I see her most weeks, two times, I walk to her studio and walk back which is 7 miles and I walk at 4 mph and when I'm there I do 30 minutes intense interval training, on top of this I also do another two hour walk within the week, now I hate exercise with a passion, but have to admit, I'm coping with this, enjoying might be too strong a word but I can see myself keeping this up! Anyway my confusion lies with the food, I know all the right foods to eat and luckily these are the foods I enjoy, however the weight loss Is slow and even my trainer was surprised at how little muscle I was building,when I've checked different websites they all say I should be eating a lot more calories than I am ( having about 1200) but I'm of the old school, where to lose weight you have to eat less, not more?? How can eating more make you lose weight, I'm struggling to get that concept in my head! Please is there anyone out there who can clear things up for me, any advice would be gratefully received! Sorry if this post had dragged on!!!!