Briefly??.....me....

?......HAHHAHAHAHAHHAA!

Hi Gosia...I'll try and answer you by starting with some background. I've only recently resumed (serious) training after decades lay-off.
I was heading in ten different directions at once over the last year or two trying to decide what I wanted to achieve..all of it in hindsight half-hearted. So I got down to specifics. My specifics are (1) To build a solid foundation of Aerobic Fitness (using a treadmill) and (2) To gain muscle mass (using high-weight-low-repitition approach as outlined by Les/Jedi and Metamorphisis in other threads).
This is the third week of switching to the les/jedi/meta approach and my 6th week overall and I'm seeing some solid results. I weighed myself as a baseline (70kg...I have been as high as 90Kg fit/muscle but that was 30 years ago) and am not planning on weighing myself again for 3-6 months.
It is now my view that people trying to lose or gain weight should stay away from scales and focus on the
daily activities they have chosen to achieve their goals until it becomes part of your every day without a single thought.
I have other very specfic things I want to incorporate into my training over the next 6-12 months and then some very specific things after that....but I'm totally relaxed about it all and focusing on the day...not some distant end result.
Which brings us to
you ....finally....hehehe....
You'll read/hear/see and get a lotta contradictory advice when you go looking for ways to train/excercise. The more specific you are about what you want to achieve the better because different types and ways of excercising can be counterproductive to specific goals.
You're swimming right?...great excercise....it isn't intense (burns way less calories say than running and you gotta do a lot more swimming than running to achieve the same aerobic benefit). I know you have an injured back....my point is about intensity.
If you want to tone up and get some muscles happenning you have to increase intensity. Intensity can be increased in two basic ways with weights with different results. Heavy low rep or light high rep. There's a third factor in there as well which is the speed of the movement.
Intensity with aerobic excercise taps into different systems....and there's basal metabolic rate to consider with regards to fat burning.
It can all be done and no doubt quite simply.....
How do you want to train?
At home?
At a Gym?
How much knowledge do you have about how muscles work and which muscles do what?
How much do you know about your back injury....how much would you or do you trust a personal trainer at a local gym (They are not all created equal).
Bottom line is Gosia...you can achieve what you want to achieve regardless of your injury...I'd need to know a lot more about it and your knowledge of physiology etc before I offered any opinion (and I realise I haven't been asked for one...I'm just trying to throw some things at ya to get ya thinking).
John