Running

No reason, just be careful building up too fast. And it does get a bit easier (if you keep it easy) otherwise it doesn't get easier, you just get faster. (But if you want to get faster faster then you need to do more stuff easier, hopefully you'll see what I mean...)

I do my 12km commute once or twice a week, but at a lovely slow "easy run" pace (which starts off feeling easy but eventually feels vaguely uncomfortable).

The general rule of thumb is not to add more than 10% distance a week, and the vast majority of your miles are 'easy' runs (at aforementioned slow/shuffling pace).

Obviously you need some sort of base to start on (as 110% of nothing is nothing) but if you're doing 30k a week (if you've done 2 x 12k in a day I assume you're happy with at least 30k a week) then 10% a week adds up very quickly (starting with 30k/week you'd be at 200km/week in 20 weeks at a 10% increase a week). Don't push it too early otherwise you'll risk injury.

The pace calculators here: https://www.mcmillanrunning.com/ are great. I put in my recent 26:16 parkrun and it tells me I should be doing easy runs at 5:59-6:33/km and long runs at 6:00-6:47/km. I'm more than happy with this as it is very achieveable, if anything it feels too slow, but it's doing me good. My commute in today (just shy of 12km) was at 6:17/km and my main goal was keeping my HR down rather than pushing harder and going for a faster time. I am seeing my times go down, but also my HRavg is going down, so I'm getting fitter/more-efficient.

I do two 'quality' runs a week:

• 40m tempo run (according to the bupa marathon training schedule) which I actually do a 5k parkrun (in ~26 minutes, aiming for ~22mins eventually) on a Saturday, and the extra time is the warmup/cool-down (which I do cycling to/from the parkrun)
  
• 40m speed session on a Wednesday (based on the suggestions from PhilPub somewhere up above; 800m, 1000m and 1600m intervals slowing building up and paced based on my most recent 5k parkrun; 800m at about 5k pace, 1000m/1600m at a bit faster than FT pace which you can work out from 5k pace
  
  

One 'long run' a week

• Currently just ~10k, but this soon builds up to 14, 15, 19, 21, 23, 15, 19, 23, ... 35k, before a two week taper and then the marathon.
  
  

The rest is 'quantity' at an easy pace, and it surprised me just how easy this pace is. No bonus points for pushing faster as that would just adversley affect my long or quality runs. These are the runs you want to add to if you want to bulk up your week, don't be tempted to add anything else that's fast.

If in doubt post your plan and let someone far more experienced/qualified than me comment on it. Doing so certainly helped me.