Can't eat enough food

Get the almonds bought!

I like the wasabi spiced ones.

But those are expensive options, but a few go a long way for calories.
Perhaps a serving of that and some peanut butter and milk.

If you've been very nutritious up to this point, probable no vitamin or mineral deficiency, just calorie if you were to keep it up day after day, and sounds like you could if not hungry enough.

And yeah, that deficient, forget about building muscle, which really needs a surplus.
Effect if constant is you'll notice it's harder to make progress, tired more often, might see Fitbit steps go down as your body tries to be lazier to adapt to too few calories, ect.

And unless you correct the Fitbit for it's vast underestimate of your lifting - you are already eating at a deficit.

I hope you are logging the lifting either on Fitbit as Weight lifting or MFP as Strength training. It may not seem like much compared to cardio - but it counts and adds up.

try some healthy nuts or nut butters, or some lean proteins, chicken or jerky.

You're not going to build muscle if you're as under as you say you are. In fact, you will continue to lose weight, which will be both muscle and fat. When you eat too little for a long period of time, especially with quite a bit of cardio, your body will eat at muscle for energy. Yes, you will lose fat as well but you lose muscle too.

There are people who eat at a deficit and weight lift, but they do that to keep the muscle they have.

If you want to build muscle or "tone" or whatever we call it, then you need to eat at maintenance or slightly over. If light to moderate walking puts you at 2000, then you may want to consider eating between 2000 to maybe 2250 for muscle gain. And this will make you gain a pound or two a month, but it would be muscle.

What's going to continue this way? You will lose weight. Like I said, both muscle and fat and water. You will get smaller, but not necessarily more fit.

Just depends on what you want. If you wanted to lose weight, then that is your decision, but still being 900 calories under is still a little much if you're looking to drop some weight. In the short term it works really well, but in the long term it leaves you hungry, more prone to binge and regain weight back, and the loss of muscle and lowering of metabolism.

You may want to plan your meals out in advance if you want to get more calories in. Peanut butter, avocado, ice cream ( of course! ), nuts and seeds, whole grain bread/muffins/bagels. etc etc are all decent sources of calories. You can treat yourself too every once in a while with ice cream or a cookie.

If it's at the end of the day and you have 900 calories left, maybe a bagel with peanut butter would be a good choice. It isn't a whole lot of food, but a lot of energy packed into a little item.

At the moment i am trying to loose some weight mostly eating veg, lean meats & fish but am finding it hard to keep up with calories without going over my Fat/protein intake limits set by MFP. The advise i am seeing on this topic is that is ok to exceed my macro limits, as long as i make up calories is that correct?

I was having a bit of trouble with volume since I went mostly natural healthy foods. Today I had 50g of mixed nuts fruit and seeds with extra walnuts. About a handful, 300 calories :-)
Try a big breakfast breakfast and if you can't eat a big breakfast make a big smoothie, banana honey full milk a few oats maybe, probably going to be 500 calories in one glass.

Well, that's just going to bend in the tub of ice cream unless it's soft already, or a well designed heavy duty spoon for ice cream eating.