Consistent 74/75 is really good. But I don't think the improvement is much of a surprise, it's down to all the training you've been doing, it's just taken a long time to work - but it's clearly working. Keep at it.
Very true. Miles slowly add up and even without too much specific speed training it can slowly improve. Some consistent speed sessions e.g. minutes, 2 minutes etc. could lead to more improvement so the new sessions your are doing are good. Combine them with an MDC hill intervals session in the winter and you will be good.