What is the best (and quickest) way to download the full archive (from 1979) of runoff from reanalysis-era5-single-levels?

I try to do the following in a simply little python script, and the API (or queue) takes days to get a month of runoff and sometimes throws errors or stops incomplete.  Is it because, in part, much of this data is offline on tape archives?  Is there something else I might be doing wrong or I could be doing differently?  Is there another way to get the data if the API, queue, tape drive etc are simply unavoidably slow?

import cdsapi
c = cdsapi.Client()
for year in range(1981,2018):
   for mon in ["01","02","03","04","05","06","07","08","09","10","11","12"]:
        c.retrieve("reanalysis-era5-single-levels", {
           "product_type":   "reanalysis",
           "format":         "netcdf",
           "variable":       "runoff",
           "year":           str(year),
           "month":          mon,
           }, "runoff."+str(year)+mon+".nc")

I have gotten the following error, as well (but not consistently):

“Exception: Download failed: downloaded 11324683 byte(s) out of 757926348”

Thanks very much for any advice.