Going for a lower gear, higher cadence - on a flat road you should feel you are exerting only light pressure on the pedals - is a good idea, it'll be kind to your knees and give you a better cardio workout.
IBOHUNT knows what he's talking about. Forget about speed. Just keep increasing the length of your rides, and eventually speed will come.
If you've ridden 150 miles in June at an average of just under 10mph, that's about 15 hours on the bike. Make your July target 20 hours - say, three one-hour rides and one of two hours each week. Then in August, shoot for 25 hours.
And congratulations on the weight loss.