Running faster

Bet your time is under an hour first time you run outside

I'm impressed you can run on a treadmill for 75 minutes without going stir crazy! I was gong to say try outside but see lots of others already done so! You could try getting to a parkrun? It's only 5k but you might find running in a group gives you the push to run faster, try to catch person in front etc.
Just be careful about pushing up mileage too much too soon as you will end up injured, so don't leap up to 2hrs immediately! I also don't think you need to run 4 times a week, three is enough especially as you are doing gym work on top.
Longer runs at a slow slow very easy pace, shorter runs/intervals push the pace, hills also help! During these winter months I run up and down my road, fast from one lamppost to the next, slow to the next, fast to the next. It's a super easy way to do fast intervals without needing to program a watch or keep an eye on a timer.

A lot depends on how hard you are working when doing 10k in 75 mins - if you're really struggling at that pace it will obviously take longer than if it's an easier effort. If your goal is to run a 10k in an hour, that will require running 6min/k - can you maintain that pace for a minute or two now?

It's good to build up gradually, changing just one out of pace/time/intensity. One approach is to take the C25k programmes and do easy pace running for the walking intervals and increase to target pace when they tell you to run. Once you can do 30mins at your target pace you can just keep building from there. There are also loads of training plans out there (I liked the Nike Run Club guided audios).

I'd definitely agree with getting outside, so much more interesting - and a friendly running club is great for motivation and support.

If running on a treadmill regularly it helps to set the incline to 1% as this makes you use your leg muscles to push forwards rather than just up and down. Finally don't neglect strength training - it's so important for long term health, and run specific workouts can increase speed and reduce injury risk.

Try heart rate training.

Loads of programs online you can follow, once you establish your heart rate zones.

It was a game changer for me.

Keep your easy runs easy and make your tempo/interval runs hard!

I agree. Outside! Mix it up. Long really slow runs once a week for stamina and endurance, a shorter medium paced run and fartlek once a week to build up speed. You must rest too or else you’ll pick up an injury and become more prone to illness. Good luck!

Ditch the treadmill!

I would get outside and just go for it. Outside is so much easier as you can challenge yourself to do a faster pace up to the next tree then take it easier on the slight downhill to recover etc. You need to build things like hill sessions in to your training and shorter, faster runs to get your keys used to moving faster.

Do you only run on the treadmill? I run much faster outside than inside! On a treadmill I struggle to get over 9.6km / hour but can do a 10k in 55mins in the park.

I think the general rule is a couple of medium paced, medium distance runs, one intervals run and one long easy paced run. You also need to work on increasing your cadence.

I used one of the training programmes from Garmin which was very good for giving you the right mixture of runs and target paces etc. but I'm sure there will be loads of free programmes on the internet. I knocked off about 15 minutes for a half marathon going from a predicted time of 2:30 to actually running one in just under 2:15.