Timestep and run time

Ran my first very simple model today, from day 150 to day 365. It is taking about 30 s per model day to run. I plan on running this model for about 60 YEARS, and as such I anticipate the computation times will be excessive. I have NDAY = 100, NDT = 1, DLTRINT=ON, DLTMAX=3600. My maximum time step already seems high compared to some of the examples I have seen, which use 30 or 100 seconds. Furthermore, reading the manual seems to indicate the model varies the time step based on an algorithm in order to maintain numerical stability. Should my DLTD always be the first day of the simulation? The manual says that DLTD should equal any value up to NDT, which is indicated to be 100 by default (and not to be changed). In the case that you run a really long simulation, can you increase NDT? Perhaps I am confused about the meaning of the timestep interval (NDT)? If anyone can provide a bit of clarification here that would be great. Thanks, Kristen