Welcome to the forum.
Try and practice breathing in through your nose and out through your mouth. It is the best way of controlling your breathing, and if you purse your lips when you breath out (like you would to give someone a friendly kiss on the cheek) there won't be much room for flies to get in.
If you are really gasping for air I would also look at how you are cycling. Don't ride in really high gears where you have to strain yourself to cycle - instead use lower gears and try and adopt a "spinning" technique where your legs are essentially moving faster against lower resistance.
You don't say what sort of bike you are riding. If it is a mountain bike it will have thick nobly tyres which will make it harder to ride on a road. You can switch to thinner road tyres which will make life a lot easier - you're local bike shop (LBS) should be able to advise. They can also advise you on fitting lower gears if you think that is an issue.
Finally it is better to build up your fitness gradually, rather than going for longer distances that leave you gasping for breath, feeling discouraged, and with a mouth full of flies. Two or three short rides a week will build up your fitness better than one longer ride.
