I was similar to you a year or so ago, had done very little exercise for quite a while. It takes some effort to overcome that inertia so well done for getting this far.
I reckon the best advice is:
1 - Don't give up, it will get easier, it will get more enjoyable and it's worth the effort.
2 - Don't overdo it, getting an injury isn't going to make you any fitter. Do what you can and increase in small increments.
3 - Mix it up, for general fitness improvements try to do a range of activities - walking, jogging, cycling, press-ups, basic weights - it'll be more interesting and exercising in different ways will be better for overall fitness and reduce the risk of injury.
4 - Set realistic but challenging targets and track your progress towards them with achievable milestones - helps to maintain motivation
5 - Don't compare yourself to others, their circumstances will be different, record your performance and use it to gauge your improvement
In your case I'd start by riding a little further each time on the flat and not worrying too much about the hill, this will help build some general stamina. See how far you get up the hill but don't be afraid to walk.
When you're doing four or five miles regularly on the flat leave keep going the same distance and work on going further up the hill each time.
It worked for me.