If r is the actual rate in miles/minute and R is the hypothetical rate in miles/minute, then, since you would have taken one fewer minute per mile,
1/R=1/r - 1 = (1-r)/r
so R=r/(1-r)
Then, according to the distances
180r + 30 = 180r/(1-r)
Solving for r, we get r = 1/3 mile per minute, so in 180 minutes, you went 60 miles.