It looks like you’re going in the right direction. Your 15 miler in Feb predicts a 3:50 and your April half predicts 3:41. More importantly it’s inline with your 5k. Too many times you’ll see a 3:41 based on 5k, but a 3:50 based on a half.
Like I’ve said here before, you can RUN a marathon on 40 mpw but I don’t think you can RACE a marathon on that mileage. I think you can break 3:50, but sub-3:45 might be tough this time around.