Using an old-fashioned safety razor with a full beard?

So I bought a merkur double edged safety razor and decided to try it out last night - the thing is that I don't shave everyday - I usually go about two weeks between shaves (touching up the neck) to keep a bit of a beard. But when shaving with the merkur I decided to take it all off - this didn't work - I lathered and made probably 5 or so pes but it just wouldn't get rid of it all - it thinned it, but wouldn't take it all off. After getting frustrated I ended up just using my electric razor to finish the job. Am I doing something wrong - or does it really just take a *long* time and *many* pes to get to a clean-shaven face from a beard?