BMR, TDEE and exercise questions

So I measured my BMR and my TDEE and they are 1685 and 2022. So my TDEE - 20% would be 1617 (less than my BMR - that can't be right, can it?). I made my TDEE at sedentary level as I have a desk job, but now I have a pedometer I see I walk 5000+ steps just to the bus and to work and home again every day, so am I really sedentary? Is sedentary level for people who drive everywhere and sit down all day? If I clocked 10000 steps per day on a pedometer but still sat at my desk all day would I still be sedentary?

Also, to put on all this weight (56lb over what I was four years ago) I would have had to be eating quite a lot over my TDEE every day, lets say 2500+. I have done a test log day for when I wasn't watching how many calories I had, and I struggle to see how I was eating 2500+ calories per day every day to make me as heavy as I am. If I logged all the high calory food I used to eat on one day it would come to 3500ish, but I didn't eat them all day every day, I ate quite healthily most of the time, with some high calory food thrown in. I ask this because I am keen to understand how, once I have lost the weight, I will maintain it?

The other thing that has been bothering me is watching calories alone vs watching calories + exercise. I am very tempted to not do the exercies, particularly as I have aback injury that sometimes makes it painful. But surely there are massive advantages to doing exercise as well - watching calories wouldn't inmprove fitness would it? Being thinner doesn't automatically mean being fitter, does it?

I am not complaining, as I am seeing great success in inch loss and my shape has chnaged, I am only weighing once a month. A lot of questions, but I am eager to learn from people who are more experienced at this than me.