You should have absolutely no problem doing 100 after doing 70. I have never ridden 100 getting ready for a 100 mile ride and that includes my 1st time riding 100!

Be aware that you will feel like you did 100! But, that is okay! When I'm riding distances, the way I get myself thru them is to ride from rest stop to rest stop. You get to the last rest stop before the end and you say to yourself - I can ride 20-30 miles (whatever it is to the end) any day of the week. I can do it today too and then "wala" you finish the 100!

Have fun, keep your sense of humor, and believe in yourself!