Great question, Chuck. My question back to you is, “When is your half marathon?” This important fact will determine how aggressively you can increase your mileage.
The good news is you’re already almost halfway there, and you also have the advantage of experience on your side. With your long run currently at 6.2 miles–and assuming you’ve been running that distance on a somewhat regular basis–adding one mile per week represents a very incidental bump in volume for a distance running veteran such as yourself.
For the athletes I coach, I like to prescribe training in four-week cycles. As a general rule of safe practice, I’ll increase the length of one’s longest run of the week by no more than 15 percent for three consecutive weeks before pulling it back a few miles in the fourth week. The body needs a chance to adapt to the increased level of stress being placed upon it, so reducing your long run and cutting back on your overall mileage every fourth week or so in a given training cycle is an effective way to do just that and will also lessen the likelihood of injury.
In your case, I’d cover 7.2., 8.2, and 9.2 miles for your long run each of the next three weeks, then cut it back to 6.2 miles the fourth week. At the start of the next cycle, pick back up where you left off, starting at 9.2 miles, then upping it to 10.2 and 11.2 before dialing it back to 8.2 miles in Week 8. On the third four-week go-around, start at 11.2 miles, then 12.2 and before you know it, in less than 12 weeks you’ve safely worked your way up to covering the half marathon distance.
Increasing your long run in this manner, combined with the addition of two to three shorter runs during the week and the inclusion of supplementary cardio work at the gym, should be more than enough to get you through a half marathon in less than three months time. Always remember, however, to listen to your body throughout the training process and don’t be afraid to back off if you ever feel like you might be overdoing it.
Best of luck!