I used to keep my lawn at 18 inches until the city made me drop it down to 12. So I have some experience with this. Use a string trimmer. Take off the top 1/3 and let it rest for a few days. Then take off the top 1/3 again with the string trimmer. Continue until it is down to mower height (5 inches) and mow it down to 4 or whatever your highest mower setting is.

Seeding northern grasses in the spring is not recommended for two reasons. One is that spring is when the crabgrass seed wants to germinate. It germinates after just a few days of continual moisture, so it's the first grass up. It will be fully established within the 3 weeks it takes for KGB to germinate. Second reason is that fresh new turf grass will not have time to develop roots sturdy enough to take the summer heat. What often happens is you end up with the new grass dying and the crabgrass taking over by mid July. For these reasons late summer or early fall are the best times to seed northern grasses.

so you are saying fall seeding allows the new sprouts to establish roots prior to the ground freezing? i seeded my curb edge and watered it all summer and it looked great all summer. than a few yrs later that grass is dead and the stuff 2' away is going strong. why is that?

Yes, grass seeded in the late summer should have three good months before the soil gets too cool. Then if the soil does freeze, as soon as it thaws the roots continue to develop at a non-zero pace until the summer heat.

Turf near concrete or asphalt often suffers from the heat even into the night. The concrete and asphalt absorb sunlight/heat all day and then release that heat in the evening and night. That heat warms the nearby soil and drives off moisture. If you have a lot of heat collecting concrete, those places need to be watered more frequently. If you are familiar with the deep and infrequent watering approach that I discuss, it is dependent on the temperature. If the soil near the pavement is warmer, then it will need water more frequently.

So I trimmed the top third and next I cut it down today to the top highest setting on mower. I probably should have trimmed some more before mowing but it turned out pretty good. It's looking pretty thick and green. A little uneven in front I think due to the trimming in some areas. I also noticed the lawnmower needed cleaning underneath which I did after which probably bogged down the mower as it was pretty thick and little wet still. Here are some before and after.

Before