I think the best way to exist at two ends of the fitness spectrum is to simply periodize your training. Hopefully you have a rubgy season, an off season, a rowing seasin, then another off season.
So one way to do it would be:
- Off-season 1: Focus on strength training, do 1-2 long rows for maintenance
- Rugby season: sport specific training
- Off-season 2: Focus on building endurance, do 1-2 short strength training sessions for maintenance
- Rowing season: sport specific training
I was a distance runner clicking off 16:00 5ks and 2:32 marathon weighing 140-145lbs at 5'5.5". Since November I've put on close to 20lbs of lean mass. As first I was still running 15-25 miles a week and still gaining weight. I then switched to mostly cycling as a means of commuting to work and running less. Anytime I run these days I still am able to jog along around 7:00-7:30 pace per mile vs 6:30-7:00 when I was a "runner". I haven't run a race since last September but I still think I can break 17:00 for a 5k even 20lbs heavier. Lesson: It takes very little to actually maintain what you've gained.
Now, if you are doing both at the same time you could do some sort of conjugate training:
Week 1: Rugby focus + strength training; Maintenance work for rowing endurance and skill.
Week 2: Rowing focus + endurance; Maintenance work for rugby strength and skill.
Week 3: Moderate deload; light work for strength, endurance and skill
So, week 1 you may strength train 3 days a week for 60 minutes. Focus on your rugby training. Then 1-2 days you would focus on rowing but using less volume and the same intensity (ie: 50% of the volume you usually do for rowing)
Week 2 you would switch it. Strength train only once that week for 30-60 minutes then focus on the endurance and rowing.
Week 3 you split the training evenly between strength and endurance but lower the volume. 3 days of 30 minute strength training, 3 days of 50% of your usual rowing training. 1 rest day.
Ultimately you will not be able to maximize BOTH endurance and strength. You will not find a 2:10 marathoner who can deadlift 500lbs and you will not find a 500lb deadlifter who can run a 2:10 marathon. At least not in the same training period.
You can however have an interesting life as a sports athlete as few people choose to try and be good at both ends of the spectrum.