It is really hard to make time for exercise when you have little ones - i have to admit I've only really just got back into it after having 3 kids in the space of 4.5yrs (youngest now 2 years old).
I breastfed my now 2 year old until she was 18 months, she started sleeping through at 12months, and whilst I tried to exercise during all that, it was pretty hopeless - i was so tired from lack of sleep / breastfeeding.
I would say:
(1) Don't set yourself crazy goals eg saying you'll exercise 4 times a week is so impossible with kids and tiredness. I just promise myself i'll get out running twice per week - one long run on a sat or sunday when DH can sit the kids, and one shorter run on a wednesday lunchtime when elder 2 are at school and my mum can have 2yo. I know I can achieve this most weeks, and if I do manage to fit in a 3rd run then bonus!
(2) Find a similar minded mum who might sit your kids whilst you run, then you do the same for her whilst she runs
(3) Find just one evening exercise class per week that you can get to - make it your weekly 'must do' eg DH must be home to babysit and you must get yourself there and complete the class
(4) Exercise DVDs are probably the best bet during the winter months - I bought a Cindy Crawford one that she did specifically after having a baby. Also, Davina does good DVDs too. Have NO guilt about sitting your kids infront of peppa pig for half an hour whilst you exercise - a happy mum = happy kids.
(5) If you're into running, enter some events? I entered my first event post kids on 29th December - 10k run through the New Forest, totally loved it! I've now signed up for 2 more 10ks later this year, just to keep me motivated. Try 5k fun runs to begin with, then work up to longer distances.
Honestly, you'll get there - it just is really tough with kids, especially a 5mo, don't beat yourself up, you're doing great just thinking about exercise!