Don't over-complicate it. Your training doesn't have to resemble what you do on the rugby pitch for it to be effective. I'm a triathlete at a reasonably high level and some of our training, you could argue, doesn't represent what you do in a race e.g doing 10 x 800m intervals when the run leg is a half or full marathon.
Download the beep test (or a similar V02 max test) and do that initially to get a baseline of your fitness. I can't really advise you on gym stuff, but running wise, concentrate on intervals and at least one long, slow (talking pace) run a week to build endurance.
There are many variations of intervals, a good simple one is 15 - 20 x 400m with 1 mins rest. These are done at around 80% and if you're fittish, you should be coming in 1.10 - 1.20 consistently. You do that 4 times a week for a month you will notice a big difference, especially in your recovery. You watch a top level player sprint the length of the field and he's all smiles, chatting away after scoring his 100m try, whereas most club players are lying on the ground slowly dying... those 400s allow you to operate for a minute or two on the pitch at a high intensity and then you have a scrum, lineout, which takes a minute or two by which time you have fully recovered and are ready to go again. I've tried to introduce a few people to this workout, but it's pretty brutal at least when you first start out. It's a good reality check though, you see guys that think they're fit do 400s and after 2 - 3 they're nailed. You can do 200s, 800s and 1km intervals for variety, but I'd build your cardio fitness around 400s.
I am a back, but personally I wouldn't get too caught up in gym stuff, it's obviously important, but the most effective back rowers I've played with have been fit, mobile and quick.