I can fully understand your scepticism..
I am indeed serving in the RM and you're well off with the requirements for just about everything. There is no speed marching with that weight for 10 miles, nor is the 30 miler done with 120lbs... It's no longer a run, more a bimble across paths.
Every Marine does not run miles - they used to, back when people thought running miles was a good idea. They run mostly because it's easier to take a troop for a run than it is to organise a circuit and get the kit. However, I have ran with my current troop all of once in about 2 weeks.
Running sprints and running hills will help you in RT. I'm not saying you can't do RT without being strong, I'm saying it will be easier to do certain tasks with a strong foundation. I'm also saying running slow miles will not help you. It won't. Running 10 miles with no kit in comfortable trainers, fully rested, in good conditions, fueled by your own nice, home nutrition, at your own pace is different to running with kit, fatigued, at the pace of PTI, trying to keep step with the mong in front of you who seems to have the stride of a gazelle with a stomach full of food provided by the galley. I do not think running miles has a good transfer. It's easy. Do 10 hill sprints with little rest. Stick a weighted vest on it... You'll see which is more challenging, physically and mentally. You'll also see which you can recover from quicker and which will help your strength goals.
You're free to train how you like, I'm just trying to offer insight into what helped me and what a lot of people who are trained ranks do. Don't get me wrong, pre RT you require a different type of fitness than what you do when you've passed out but largely, a solid foundation of strength is key.
I also spent a fair bit of time in Hunter Coy and the PTI's there actually know what they're doing and the phys is a lot different to what is recommended on the RM website. For instance I had a strength program, Crossfit was programmed and I did a lot of rowing intervals, spinning intervals, tabata etc.
They recommend a certain type of phys in the careers office and online because telling you to do weights becomes a liability for them. People can't balance their training properly and they start doing BB routines that someone in their gym gave them and focus all their time on the wrong sort of phys. Generally with weights is also an association with supplements, which they cannot be seen to condone, because young impressionable people will get the wrong idea. It's easier to recommend bodyweight phys and running, even if it is outdated and not optimal - because for most people it will work OK. But that's it - OK... Not great.
I'm trying to tell you to open your eyes to a better type of training which will give you optimal results for the time you invest into it. I can give you some decent advice, beyond the scope of this thread if you care to message me.
EDIT: I do think there is place for long runs though - just not 4 per week. 1 every 10 days or so with varying distances is more than enough. Focus more on sprint intervals and distances up to 5k for your regular running. Fast paced, balls out running. If you do 6-10 miles, use it as a recovery run at a very steady pace just to get used to the boredom of long distance more than anything. Well... I say recovery but I don't think there's anything in a 10 mile run that helps recovery - swimming would be a better idea as it's what most people neglect prior to training.