What you want to do is get a city map, look at the scale along the bottom (the little line that shows distance on the map in relation to the real life distance) and measure a piece of string that could cover a 6km distance on the map. For example, if 3cm on the map was 1km in real life, then an 18cm string (3cmx6=18cm) wil give you 6km. Now place one end of the string on the spot on the map where your house is, then place the rest of it over roads and trails, then back to your house again. Voila!! With 5 minutes work you now have an accurate way to guage the distance of your running and an easy way to figure out new routes.
So now that you have the distance figured out, you need to know how fast you should be going. 5min/kms are pretty much the standard for running, at least in 1RCR and the RCR Battleschool. So 6km x 5mins = 30mins. Now look back at your map and pick a point about halfway through your route - a crossroads, for example. Now you can look at your watch and be able to say that if you're at that point in under 15 mins, you're doing good. But if it took you longer then 15 mins, pick up the pace. Maybe, psychologically you'd do better if you divided the run into three 10 minute sections. Experiment.
The secret to improving your running is getting it through you head that you're going to cover that distance one way or another, no matter what. Run untill can't run anymore, then shuffle untill you can't do that, then walk if you have to. As soon as you get a bit of breath back, start running again. Just cover the ground you set out to do, at all costs. Don't stop moving, and after a few weeks you'll notics a drastic improvement.
Just to give you a bit of a standard to gauge how you're doing: At BMQ in St Jean, its pretty hard to fail because you can't run. The runs are on your own time, and the standard for the beep test is pretty low. If you're going combat arms, its another story all together. They started my course at about 5kms at a decent pace, then for the next 16 weeks increased it to an average of about 8kms in 40 mins. The runs get longer if you spend any amount of time in a holding platoon, but when you're actually on course you only have limited time in the mornings before inspection to do PT. As well, its common to do a run in the morning and a ruckmarch in the afternoon - expecially on the SQ. If the runs got any longer they'd be pushing the limits of what the troops bodies can take and the course staff would start dealing with stress fractures and other sports injuries. In holding platoon, on the other hand. There's all the time in the world to take you for long runs, since there usually isn't much planned for the rest of the day anyways. I was lucky enough to only spend a week on holding, and the longest run we did in that time was about 13kms in about 1:20. In battalion, alot of the PT you do depends on your platoon commander. My first platoon commander was more of a ruck march guy, and thats what we got good at. My current platoon commander runs like a fucking Banshee. Today we did the CO's Route (Down RCD Hill, a left past the Tent Pads, Up Brindle Hill, then back to Y101). Its about 8kms in just under 35mins. I'm pretty sure thats a personal best for myself. If you can do that before you start Basic, all the power to you. But most people need to be worked up to it. Thats a no-bullshit assessment from a guy who did his BMQ, SQ, and BIQ within the past year. Being a marathon runner will make life MUCH easier for you in the Battleschool, but I was doing 6kms in 30mins a couple days a week a few months before I started Basic and I've never had any problems. Just get the minimum down before you start so you're not trying to play catch up later on.
I hope this helps!!!