How can you say Miami didn't have a good running game with Miller as the starter? First off, Miami threw 588 times while running only 344 in 2015. Miller was 13th in yards in 2015 despite having the least amount of carries of all 12 RBs ahead of him statistically.
In 2014 Miller was 10th in the NFL in rushing yards despite again having the least amount of carries of the 9 RBs ahead of him.
If I was to ask you to name the top 3 RBs in the league, who would you choose? Let's look at the top 3 RBs statistically in 2014.
DeMarco Murray: 392 carries, 1,845 yards, 4.7 avg, 13 TDs. 57 receptions for 416 yards and 0 TDs.
LeVeon Bell: 290 carries, 1,361 yards, 4.7 avg, 8 TDs. 83 receptions for 854 yards and 3 TDs.
LeSean McCoy: 312 carries, 1,319 yards, 4.2 avg, 5 TDs. 28 receptions for 155 yards and 0 TDs.
Now let's look at Miller.
Lamar Miller: 216 carries, 1099 yards, 5.1 avg, 8 TDs. 38 receptions for 275 yards and 1 TD. I grant you it isn't a perfect way to estimate what could have happened but let's say Miller had run as often as the top 3 RBs in the league in 2014.
Murray ran 392 times. If Miller had also run 392 times, he would have had 897 yards more than he really had based on his 5.1 ypc for the year. That would have given him almost 2,000 yards which is more than Murray had.
Bell ran 290 times. If Miller had also run 290 times, he would have had 377 more yards than he really had based on his 5.1 ypc for the year. That would have given him 1,476 yards which is more than Bell had.
McCoy ran 312 times. If Miller had also run 312 times, he would have had 489 more yards than he really had based on his 5.1 ypc for the year. That would have given him 1,588 yards which is more than McCoy had.
None of this even takes into account how many more rushing TDs, receptions, and receiving TDs Miller could have had. Miami didn't run the ball with Lazor at the helm. In 2015, the Dolphins ran 344 times which ranked dead last in the league despite averaging 4.3 a carry which tied them with the Carolina Panthers and 6th overall (Tampa Bay ran had the best average per carry at 4.8).
In 2014, Miami had the 22nd most rushing attempts despite having the 2nd highest yards per carry average. They were behind Seattle (who averaged 5.3 an attempt). Seattle ran the ball 525 times in 2014, Miami had 399 times. Seattle played to their strengths, Miami had no clue how to do the same.