You can't 'motivate' anyone. That has to come from them. If you 'suggest' fitness routines, she'll likely take it as a subliminal message that you don't think she's good enough for you.
Also remember, fitness is one thing. Bodybuilding, strength training, crossfit, zumba, pilates, yoga...are all just ways to 'workout' to get you to your desired outcome. Whatever that may be.
Why don't YOU try going on her walks with her? Start by giving, rather than having any level of expectation.
Everybody trains or 'wants' to train for a different outcome. Hers may be very different to yours, therefore working out together might be rather pointless?
Once you've been on a few walks together, invite her to the gym, organise a free pass for her & get excited about showing her those 2.5kg db's that have her name on them. lol
Show her youtube vids on female training & remain excited about the walks with her in between your own training, to show her that you're focused on cardiovascular fitness as well as the creation of muscle & the building of strength.
Encourage her.
Get excited about her achievements. No matter how small.
You can't motivate her, but you can always influence people with positivity.
And remain open minded & flexible about 'workout routines' - no one way, is right. It depends entirely on your goals.
I swim/ride/walk/box/lift weights/do yoga as well as a miriad of different HIIT routines. I aim for good cardio fitness, increase lbm, get stronger, leaner & faster.
But thats me. Maybe her only aim is to sustain cardiovascular fitness? By walking for 30 minutes 3 x a week, shes doing just that.
When I was overweight, you couldn't have paid me, to join a gym or move my fat ass. I was ashamed, embarrassed & terrified. Walking is where I started & I still walk, just to walk. Its good for you
Its up to YOU to begin where she has started. Walk. Make it brisk, invigorating & get excited about it.
Excitement & positivity is infectious & influential.
I have every muscle bound man I know doing yoga. It was no mean feat! Influence with enthusiasm & give them opportunity to see the benefits for themselves (i.e. get her a free gym pass, you showing her a few simple bro curls or seated calf raises) encourage her, be happy that she's there. Be patient & be positive.
If its for her, she'll at least be curious to continue. If its not? She'll still walk.
No one loses