I have two cocker spaniels who run with me every time I go. I'm currently running up to 8-9 miles and they happily do this (although I'm running at about 9 min miles). This amount of running does not exhaust them, they will still go mad round the garden when we get back!
I do run with them on leads, but they are not allowed to pull - they have to run to heel. Yes, having them on lead does mean that I have to allow them the odd wee stop - but they aren't allowed to just stop whenever, its when I say so.
Mine are nearly 6 years and 2.5 years old and both have run regularly with me for over a year. They are also brilliant family pets. I will point out that they are Working Cocker Spaniels, not the show type, therefore they are bred to run all day flushing out game birds.
With any breed of dog, you need to do your research. If it is going to be a family pet, then temperment has to be your first priority, does it require alot of grooming? (my spaniels need a fair bit). With any breed, you need to spend the time training them (my youngest has not had enough training, hence can't be allowed off lead in open areas - my fault, not his) and the whole family need to agree on what is and isn't acceptable behaviour. Find a puppy obediance class and take them as soon as you possibly can.
Here's a pic of my boys taken a few weeks ago 