Molly, apparently breastfeeding alone uses 500 calories a day!
Also you get really thirsty (or at least I did) so you drink tons of water whilst feeding and dont end up as hungry iyswim.
Plus it makes your uterus contract so it goes back into the proper shape quicker.
Breastfeeding itself, ime, isn't hard, it's the fact that at the beginning it is very tiring, because it is, at times, constant, whereas with formula feeding you're told, 'give baby this much milk this many times a day'.
Once you've got your latch sorted, and especially if you sidecar or co-sleep so you can pull baby over to feed in the night, you actually a lot more sleep, so it is much easier, plus no sterilising or making up bottles etc. They both have pros and cons in that respect.
Sometimes people find breastfeeding hard, you can get sore nipples or a very demanding baby etc so if you do try it and do end up feeling like you aren't coping, talk to someone, there will be tons of advice available, it's just finding it.