Running out of breath

I have been running for about 2 years. I usually run on a treadmill and find I can run for over 60mins without getting out of breath. When I run outside on the pavement I find it very tough and after 2 - 3 kms I need to stop for a minute or two to get my breath back. I run 2-4 times a week generally. My question is how can I improve? I am nearly 53yrs and don't want to run longer than 10km events but would like to run them non -stop. I don't like running fast...just jogging along.
I don't suffer from any injury or muscle fatigue, just run out of puff!

Run outside. And slow down. I know you say you run slow, but run slower if you have to.

There are lots of Bridge to 10k plans that can help you with this.
Most start at intervals of running 10 minutes/walking 1. If you can do 2-3 k, you can probably start here. If not modify it to run shorter time intervals at first and increase each week.

A treadmill kind of forces your forward and at a constant pace... outside has changes in elevation, wind, and you have to propel yourself forward. Improve by running outside more than the treadmill. Throw in sprint intervals for increased speed/endurance (towards the end and only like 30 seconds at a time will help). And just give it time, slow down if you have to, but don't be afraid to push yourself!

May sound odd but have you been checked for hayfever / asthma - you can have asthma without an obvious wheeze and which can be late on-set - I know people not diagnosed until into their 50s. May be worth checking just in case and doing lung function tests if it's happening when outside but not in the gym.

Thanks all for your suggestions. I did the C25K and the B210K. I have run a few 10km events, some non-stop but this year I just can't seem to do it!....maybe just getting older is the reason.
I can't run much slower than I do really otherwise it would just be a fast walk.
As most say I probably just have to run outside more often to get better. Its just that I tell myself its better to run than not, then choose the treadmill!

My thoughts

As above running is easier on a treadmill and

1) Have you lost weight since last year? Are you eating anything different to last year?
The reason I ask is that if you were heavier last year although in theory that makes running harder it might also mean your body had more fuel to burn. And equally if you were eating the same but heavier or if you are eating different now.

2) There have been issues in the UK recently with exceptionally poor air quality could it be related to this? The poster who mentioned asthma and allergies also makes a good point - worth getting checked

3) Do you only run? It may be worth building in some strength work for your legs, stronger muscles are more efficient - think of the olympian sprinters, those legs are not just built from running, they are going to be doing some serious weight training too.

Conclusion
You may need to shake up your diet. You may need to consider air quality, pollution and your own lung function. Maybe instead fo running on the treadmill you could do some weight training or bodyweight stuff to your routine.