IFR flight plan filing question

working towards my instrument, only about 7 hours so far, but taking the
written next weekend and feeling pretty confident about everything so far.

question i had planned to ask my instructor, but maybe i can get an answer
today...

suppose i'm flying from denver to rapid city on a VFR flight plan, or no flight
plan at all. i check with FSS about conditions at RAP, and they have changed
to IMC, or forecast to change to below VFR mins at arrival.

could i then file an IFR plan with FSS from the air?, head to the nearest
airway and switch to ATC?

if yes, i could then open with ATC from wherever i'm at? and if i was on a VFR
flight plan, would i just be changing it to a composite flight plan?

when my VFR x-countries i usually fly direct, but i always use flight
following. i've read posts about problems with filing direct- but i'll save it
for another time :)

thanks in advance

james...

<<Not from Denver to RAP IFR you won't. You'll be lucky if you aren't
half way to Goodland by the time you're turned northbound.>>

i fly out of front range, so i'm halfway there already:)

thanks for the responses - exactly what i needed to know.

looking forward to the IFR rating - my main reason for flying is for efficient
transportation - to get to places i like to visit in colorado or around the
area, and not so much to build hours. and hopefully allow me to fly longer
x-countries without planning the few days contingency of being stuck somewhere
- but will take it cautiously at first of course.