Hi gogolegs,

There are a lot of forumals out there to assist with speed training, but it sounds like you are doing a great job with just being consistent and slowly trying to push out your miles in combination with a walk/run routine.  Remember that overall endurance naturally builds an increased pace over time.  However, 400 meter intervals might also be of assistance, so try those with about a 1.5 minute rest period in between.  You could do those instead of longer routes on certain days and try to pace yourself at a higher constant speed for 5 to 7 repetitions.  Also, hill work is of some benefit and will improve your overall fitness, even in shorter sessions or combined with other efforts. 

It is easy to become impatient with progress...but it will show up when you least expect it!

P.S.  Enter some 5k events...they are great for motivation and usually fun events to keep the focus. 

Wishing you all the best.