Running Fortran 90 (F90) code in a Fortran 77 (F77) compiler requires significant modifications. The code must be adjusted so that each line starts in the 7th column, which is a requirement for F77. Additionally, F90 intrinsic functions, such as MATMUL for matrix multiplication, need to be rewritten in F77 syntax, often resulting in the creation of nested DO-loops or custom functions. The complexity of the task increases with the amount of code, whether it's 1,000 lines or more. There are also considerations regarding whether the F77 compiler supports freeform format, which could affect the conversion process.