I'm with Lucy and the walking although I think 1600 cals wouldn't be enough for me. I found that planning what I will eat makes a big diff and ensures that I eat properly. I plan sort of like this:
Breakfast
Porridge or
weetbix or
baked beans on toast or
eggs on toast
with a piece of fruit

Morning tea
crackers
yoghurt
fruit
crumpets

Lunch
Tuna and cott cheese on crackers
Vegie soup (I make a batch every Sat to do me for the week)
Salad with four bean mix

Planning then takes away the "what will I have?" and leaving it till I am starving and eat anything. Same with snacks- have them ready. Portion out dried fruit, nuts, crackers etc in little bags, wash and cut up salad and fruit for the day so it is ready etc. I also plan every evening meal for the week and use it to write out my shopping list so I only go grocery shopping once a week.
Also, I make sure I eat some leafy greens every day (with dinner or salad at lunch) and fresh fish once a week and tuna or salmon a few other times as I think these are important for breastfeeding and brain development. Nuts are great too and very nutritious.