mysample <- mydf[sample(1:32619,25),]
mylm <- lm(spd66561~spd66305,data=mysample)

spd66305 spd66561
4        77       77
9        78       76
10       82       82
14       76       75
21       72       74
23       79       77
25       77       77
27       76       76
28       77       78
30       78       77
32       75       76
33       78       78
34       77       81
35       77       78
38       80       79
39       77       78
40       78       78
41       78       78
42       81       80
43       77       78
44       82       79
45       81       79
47       80       80
48       78       79
49       79       78

for (i in 1:200) {
  x[i] <- (mydf[sample(1:1000,25),])
 }