18 mph on the bike is good considering this is just your fifth. What really improved by overall times was swimming with a club and getting my technique down. Of the three legs, getting proper technique in the water really makes a difference. Watch some videos and have someone critique you.

Your run times will get much lower too with adding distance and going with a group if you can. Do some runs at an easy pace longer than your tri distance.

If you want to get down to a 20 mph cycling average, do accelerations on you rides where you average something like 21 or 22 for short periods. Progress comes slow but keep at it