The cost of weight (running)

Since last March I have been doing something my body hasn't done in several years, gaining weight.

I have been 12 stones, 168lb, 76.2kg for almost 2 decades within few pounds. In March I was exactly this with 15.something% body fat
In late August I weighed in and was 12 stone 8, 176lb, 79.8kg expecting this to be due to bad diet from a long term house guest we were helping out.
Beginning of the year I weighed in again and was 12 stone 12, 180lb, 81.6kg, which I assumed to be due to excess over the holiday season. The weekend just passed I decided to find out my body fat and found I was 12 stone 13, 181lb, 82.1kg at 16.8% body fat.
As you can tell I don't weigh in often, and the weight is not excess I had expected but predominantly lean mass. This was a serious shock, to an ectomorph used to working incredibly hard to gain a tiny amount of weight especially when my commute is run or cycle.

Weights have been higher than for a while so this shouldn't have been as shocking but I'm not always as smart as I like to think.

Another thing I rarely do is time my commutes because there are too many variables, with traffic etc. So I assumed my normal 8mph with occasional increases toward 8.5 were being maintained. I decided to find out how I am doing so strapped on the stopwatch and tried a few different route maps for accurate distance.

Stopwatch no problem, nice accurate 38 minutes 42 seconds.
Distance, less simple, sites gave readings between 4.6 and 5 miles. The most likely said 4.75 which is around average, so will do. I don't have fancy GPS.
This makes my pace 7.36mph, aka annoyingly slow. I will give myself some slack due to snow and ice as long as it picks up to over 7.5mph over the next month. Slowing a bit due to additional bulk is inevitable but there is a limit to what I will accept.

I will be logging the 3 trips a week I run, which matched to the change to 10, 8, 6 in the gym could spell disappointment for me on running pace. See how I go.