Back after a couple of days with the news of the first steps I've taken following some of your advice (I intend to follow more, but I'm starting little by little so as not to get discouraged).
1) After dragging the under-desk elliptical from my home to my workplace (in the process of which I'm sure I lost millions of calories and a couple of fat rolls, if sweat means something), my boss told me there's no way I'm going to use that there, even when I'm not seeing customers, as those at the other desks would be distracted, with just a transparent partition between us. I don't really know whether it would make noise so as to be noticeable outside my cubicle. Now I have to decide whether to return the elliptical or keep it at home (where I could use it at the weekend).
2) I decided that Friday would be the best way for the thirty-minute walk to the next bus stop on returning home, since going to bed later is not so critical (I work on Saturdays, only later hours). I have to say I feel great after doing it. I have made up my mind to do other walks (from thirty to sixty minutes, if I can afford) at the weekend too. But I wonder whether I'll have to wait until next. The blisters on my feet look like I won't be able to walk much tomorrow. It's sad, after all I've walked (the only exercise I've ever really enjoyed - when I was a teenager and on holidays, I used to go to places 6-7 miles away from my home, have a picnic there and come back, just for the fun of it). I don't want to walk in a weird way because of the blisters, as I have lumbago (which is what made me stop going for walks, as anything over 15-30 minutes will make my back hurt), and it might get worse if I don't walk properly.
3) I tried not to have breakfast one day, and by 10.30 am I felt as if I was going to faint, so I ended up going to the vending machine and buying a Mars bar, the sort of stuff I hate eating (there's not much I can buy there). Having had diner at 9 pm, I really couldn't wait until my 2 pm lunch break with an empty stomach. I've almost discarded this, although it might be a matter of getting used to it (if Muslims can fast for so long during Ramadan, I suppose everybody can do the same).
4) I'm trying to reduce the amount of food I eat by leaving some in the plate.
5) I've also started to write down everything I eat on the My Fitness Pal website (I cannot use apps on my old mobile, the old Nokia type). I'm finding this difficult, since everything seems to be very approximate (there's no way I can know the exact calories according to the specific recipe they use at the restaurant, and I don't even know the weight of what I eat).
Except for the two days, the number of calories I've eaten is lower than my daily goal, but this daily goal increases every day, which I don't understand. And then, when I look up the net calories report, there's a red line with a daily goal which is the same every day. As if this weren't enough, my news feed says I haven't reached my goal. I'm puzzled about all of this, and I suppose I'll end up asking help on the forums. I don't see how this may help me, honestly, but I'll carry on writing down everything and see what happens in a couple of months.
Thanks a lot again to everyone! I'll visit this thread all the time, so I can add other things little by little.