I would second the suggestion of seeing a physio and explain what you're trying to do and see what they suggest to avoid aggravating your neck/back. You can then see a personal trainer if needed, but only after you have a set of 'don'ts' to avoid injury.
Swimming can be good, but if your form is wrong it can be one of the worst things for backs/necks. Also, if you're wanting to keep bone strength, gravity is helpful, so swimming can be less helpful as you need the force to work against to stimulate that bone density.
I've had to change exercise with age and to take account of hypermobility, and now have an hour ish routine which includes warm up on treadmill, body weight exercises such as press-ups, jumping cardio (e.g. star jumps), weights machines (e.g. leg press, knee lift, lateral pull downs etc) and then specific physio exercises for necks and also pilates core exercises (e.g. teaser, pike off a ball) and then stretches for backs (spp. sacroiliac, as that really helps). I've been doing it for many, many years though, so don't start there.
My advice is also to be patient and don't push the speed of weight increase etc. too quickly. Try to avoid the injuries as they just put everything back. Older bodies need to be reminded that they can work and once that's in place, they can do most of the things that the 20 yr old ones can (mine is mid 50s) x