The ANSI/TIA-758-A-2004 standard known as Customer-Owned Outside Plant Telecommunications Infrastructure Standard offers the following guidance for the distance between handholes or pull boxes;
"184.108.40.206.3 Lengths between pulling points
The section length of conduit shall not exceed 183 m (600 ft) between pulling points."
If you are only planning to pull optical fiber cables, a common industry practice is to place the handholes or pull boxes up to 450 m (1500 ft) apart. Regarding the pathway choices, the use of galvanized steel conduit is not a common practice in such installations. Many underground installations of this type, with the exception of under road or under rail, typically use schedule 40 PVC or EB 35 concrete incased pathways. The choice of pathway is of course based on many factors and you may have already determined the best choice based on your options.