This topic is relevent to competitive running.
One figure I’ve seen generally quoted is 1 second per mile faster for every pound that you weigh. So if you run 6:30 per mile now, dropping 20 lbs should put you at 6:10 per mile in your 5k. Not everyone subscribes to this logic so take it for what it’s worth.
There is a an article I’ve read on peak performance online that is worth reading.
Here is the url: http://www.pponline.co.uk/encyc/0048.htm
I’ll also tell you that you will lose weight training for a marathon. I find that long runs cause me to drop lbs. This may have something to do with burning fat as opposed to carbs but I’m no expert.
One last bit, find the weight that is right for you. Lighter is not always better. I think the distance you plan to focus on is relevent and worth considering. There are successful milers your size.
I’m six feet and weigh 159 currently. I’m in week 5 of a training program and will likely weigh right around 150 when I race at the end of April. I find this to be optimal for me in the marathon.