You have a fine start. The concept of specificity says the only way to improve at running is to run, which you are doing. To reach the marathon level, you will need to gradually increase your maximum distance.
Summer is the season of 5k (about 3.1 miles) running events. Check your local newspaper's social and sports calendar to see when these races will be held in your area. Sign up for a few of the 5k events. This will give you some much needed "speed work," plus it will introduce you to the thrills of registration, packet pickup, parking, bib numbers, starting lines, pacing, water tables, finish lines, and seeing your name and time on the web. If you enjoy these events, as you should, then you can move up to 10k, 10 mile, 1/2 marathons, and full marathons in future seasons.
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